The browser's main thread is expensive
kciter.so
kciter.so
The issue is though that it's too focused on interactivity. In reality, 90%++ of slow sites are not slow because of interactivity really, they are slow because they ship enormous react/nextjs bundles and have extremely heavy hydration work to do.
_so many_ sites have bundles >10MB that need to be downloaded, parsed and hydrated.
I've even seen (many) sites which have multiple SPAs stacked inside of them.
If you're on a slow internet connection and/or CPU the page is basically unusable for many tens of seconds and no amount of yielding post bundle hydrate will really solve that.
Stop building SPA, go back to HTML. Re-assess every third party. For extra performance and scaling, implement cache. Relax and see web experience healing.
Especially when trying to buy something. I much prefer to have a longer initial load and then have everything just work instead of waiting as I navigate between pages, the multi stage checkout, confirmation etc.
But yeah, if I'm just trying read a single article on a blog, preloading everything is pointless. A hybrid site with the initial page being server side rendered + progressive enhancement afterwards is theoretically optimal in my opinion.
Ever submit some giant form to get some random error and then lose the entire state of it? Used to be extremely common, even though it shouldn’t have been.
Use the appropriate technology, everything doesn't have to be a SPA.
If that's a connection-less interaction or something small that stays on the same page, sure it makes sense to keep it in the browser. You don't need to build your whole site as a bloated SPA just for few limited use-cases.
A prime example: cookie dialogs. On a slow connection, page loads, large parts are rendered, and you start reading. After that, some script starts to present a cookie dialog, and everything freezes. Page doesn't scroll anymore, buttons don't work, sometimes a previously-readable page is darkened or otherwise obscured so you can't keep reading while this goes on.
Then a whole bunch of stuff is downloading, which (again: "slow connection") takes forever. Like 20s+. Halfway through you see "accept / reject / settings", but none of those buttons respond (except the dark pattern where "accept" often works faster or smoother than either "reject" or "settings". Aaargh!). When things respond again, consider yourself lucky when page re-renders as before.
Which also runs afoul of 1 of my pet peeves with user interfaces: DO NOT PRESENT A UI ELEMENT UNTIL CODE TO PROCESS ITS USE, IS PRESENT IN MEMORY & READY. Really simple right? Yet I see examples ignoring this oooften.
A 'slow' CPU, low RAM/swapping etc just makes this worse. Web developers tend to have fast machines & connectivity so they may not even be aware of this. Or think it's a non-issue even though it affects many users - existing or potential.
And in London many buildings are declared as historic making it hard to get a permit for any work. The best chance is to wait until old pipes bursts and digging has to be done in any case to put fiber along the pipes.
You'll notice buildings in central London that are listed _but_ have housing association ownership nearly all have fibre to each apartment, as they did portfolio wide deals with hyperoptic etc.
And pipes don't help. Openreach (who owns the copper network) are not allowed in 99% of cases to "fix" copper with fibre under the agreements they have with building owners, they can only make like for like repairs.
The worst affected apartment buildings are 90s and pre 2015ish. Everything after that got fibre installed at build time.
Btw it is worth checking if you have an altnet like hyperoptic, community fibre or g network available. The majority do and if you are just checking for openreach or VM broadband it won't show up.
> 【Graphics】 Intel Graphics
And this is why you don't buy laptops from Amazon. At best, the seller has no idea what they're doing, at worst, this is fraud.
It will certainly help to download the bloat faster, but you've still got to do all the hydration as OP put it.
Based on my experiences with windows 11, that laptop seems like it will be quite a bit worse than the one I was using, even though mine probably has a much worse cpu.
> afiori: JavaScript's execution model uses cooperative concurrency to avoid a tons of data races, preemptive non-parallel concurrency would make all web development incredibly harder
How's it going to do that? What's so bad about it? Especially in the multi-core era which lets multiple programs run at the same time the way that multiple pages can run javascript at the same time. The lack of preemption is only within a single page/program.
During periods of time where no single chunk of code runs for more than a millisecond, cooperative multitasking should have the exact same performance as preemptive multitasking. And even when single chunks of code do run for that long, if that code wasn't properly isolated then preemption doesn't save you from lag.
So the difference only shows up when you have chunks of code that keep running way too long, that are also inside of properly isolated threads/callbacks, and on web those threads/callbacks are not inside web workers. Is that a common situation?
Of course, doing everything in one thread most of the time makes everything easier, because you're using the thread as the lock, so to speak. But using locks is bad for performance and latency and thus UX.
Also look at single core focused systems like spacetimedb, sometimes if a workload is not embarrassingly parallelizable multithreading can make things slower.
JS needs some way to more easily push things off the main render thread, but i think that there is no easy solution for it. (maybe a model like iirc clojure had where "threads" would automatically rollback and retry in case a data race was detected could work to handle off-threads tasks in js, but it is a quite far off solution)
That is because the latest framework always fixes everything that wasn't broken before.
React enjoyed its spot in the limelight. Then, unskilled people wrote bad React and the framework, not rank-and-file developers, took the credibility hit. It is more difficult, I suppose, to make a user-perceived slow webpage in jQuery.
I dispute this. I don't have any statistics either, but my subjective experience is that most websites are not like newspapers or blogs. Even shopping websites have pretty heavy interactivity these days.
Also, most boring pure-information-presentation problems are mostly solved by ancient technologies like wordpress. If you're working in web development in 2026, you probably aren't making static websites or blogs. You're doing something novel that probably has much higher interactivity demands.
HTML is abysmally lacking for any interactive (which is what this article is about).
Decades later, even something as common as a combobox is unsupported.
EDIT: Downvotes but no argument.
Documents vs Programs
The parent is rightfully pointing out that most websites are documents and have no need for client side rendering or interactivity. You are rightfully pointing out that VanillaJS is insufficient to build software in the browser.
Where I imagine you lose some people is that comboboxes can be done natively with the datalist attribute.
Where does that html come from, then? Do you honestly believe all pages could be static html+css served from some bucket? Or do they need to be rendered by a programm running on a server? Because once you start talking about servers generating pages then all this talk about JavaScript frameworks boils down to arguing where the complexity should be in place A or B.
But even given your position you see no difference between shipping the user a binary and shipping source code plus a compiler in terms of the user's experience? Surely the existence of server side react points to the server doing the initial lift being an improvement.
I don't think you understood my question. I stressed the fact that complaining about the complexity of a page implemented with a JavaScript framework is a red herring, because said complexity doesn't go away by moving it to a server.
And the old "dynamic HTML" approach is not easier to maintain and developm. By far.
In the meantime, what goes away is performance and perceived performance. Your dynamic HTML pages need to travel all around the world until clients see an update, and a page reload is far heavier and time consuming than doing a fetch to get data.
Try to ask yourself this simple question: why do software engineers bother with JavaScript frameworks? Do you think everyone has absolutely no idea about what they are doing?
> said complexity doesn't go away by moving it to a server.
> why do software engineers bother with JavaScript frameworks?
Some of that css and html doesn't run reliably and consistently on all browsers. A visit to sites such as canIuse helps build up an idea of the extent of the problem.
Also, you seem to ignore the fact that JavaScript frameworks use said html and CSS extensively, and provide the necessary abstractions to bridge the unreliability gap in addition to introducing features that html and css do not support.
What I really find funny about this issue is the fact that this type of claim implies that virtually all software engineers who for some reason aren't html+css purists are utterly incompetent and completely unable to assess any technical tradeoffs. Imagine yourself walking around with the belief that an entire field is manned by people who don't have a clue about what they are doing.
> For most of us it’s things like reducing network requests, shrinking the bundle, or making good use of the cache.
These are likely going to be your first port of call for performance issues at your day job, but fixes like removing dependencies or making fewer network calls are pretty straightforward. I think the author chose to do a deep dive on the topic on freeing up the main thread because there's such a wide variety of approaches and many of them may not be obvious.
Also, removing dependencies is not easy. I've seen many corporate that have a huge UI lib for example that everyone should use for brand consistency. But it's many MBs of JS, because it has to cover every possible use case.
This doesn't even get into 3rd party vendors who _also_ ship react et al and have other bundles.
I'm not saying the article is wrong, but if you want to free main thread time especially at the most critical point (when the user has initially loaded the page) you _probably_ will find that most of the opp is in bundle size and hydration improvements.
But yes, I think we are in agreement :).
Can you tell I have mental scars from nextjs bundle optimisation?
The web is much faster when useless JS is taken away.
There was very little new information for me as I have applied some of these techniques myself based on having developed an intuitive understanding on how a rendering "thread" works and blocks, but for a less experienced developer this article should be incredibly enlightening and provide a solid understanding of what's happening.
I employed use of yielding on a hobby project [0] I made about 15 years ago, particularly when it had to do lots of draw operations on a canvas. I also experimented with using worker threads to render pieces of it on a background thread, but at the time there was no way to copy the data efficiently between them and the main thread, one had to send the data as a base64 encoded PNG and the overhead of encoding, decoding and then copying it onto the canvas made it perform far worse than just doing it all on the main thread.
The website also does Gzip decoding of uploaded files in JavaScript and the library I found at the time did all the work synchronously so could lock up the UI thread easily for 10+ seconds. I tweaked it to be able to yield every 200ms or something, the process of which was very educational, particularly due to it convincing me to never omit the curly braces after an if statement, I spent a very long time trying to understand why it wasn't working until eventually I realized a statement I added wasn't in the if statement's block. It's not that I didn't understand how if statements worked, it's that in my mind the lack of curly braces was initially invisible to me.
It isn't absolutely necessary to match the display refresh rate. With a 144Hz monitor, 72FPS is going to look smooth for the vast majority of the people, and even 48FPS will look smooth for most people. I agree that higher FPS is better, but there are diminishing returns.
But it's more about honoring the user's preference. I personally would rather 60Hz, then 120Hz heating up my room unnecessarily. If someone has a display set to 1000Hz for whatever reason, then you should try your best to honor that.
But importantly, if your screen is variable refresh rate or high rate enough compared to your typical performance, that greatly reduces the inconsistency. Go ahead and vary between 55 and 70 if the screen is at 165. Definitely don't drop to 30 and I don't think you want to drop to 41.25 either.
If you need to read and write from both threads at once, you need to look at SharedArrayBuffer.
the whole point of cooperative multitasking is to yield now and then (back to the runner), so that it can process the drawing. Besides, at 60fps there's so much that can be computed once every N frames, and the eye does not see it, the animation flows.
It becomes even more interesting when webgpu is involved, but the CSS animator is indeed very fast.
note: some recent work of mine (newskool digital flyer sites) -> bsf.hmsu.org // nouveauxhivers.dub4powder.xyz
> So much of development is trade-offs, and you have to choose according to the situation, which ultimately comes down to the developer’s experience and judgment.
It’s a great article, but I think it is a bit behind the ball in framing scheduling problems as a matter of “experience and judgment”. The problem of allocating work to a scarce resource is one of the oldest and most well-studied in all of computer science. It would make sense to go consult a textbook on the matter before trying to reinvent the wheel.
I’m not sure whether inventing a setup like green threads in JavaScript/workers would even be possible.
That is not to say that we should just forget about those learnings though.
(Don't get me wrong; I too think the article is great; just wish all the folks who went to JS bootcamp and are amazed by this would have cracked any CS or OS text book written since the late 60's and browsed a few chapters.)
Yeah fuck everyone doing that, hello Reddit, Outlook for Web or Bluesky. It makes searching on such "feed" pages with the browser's search function an utter pain in the ass, made worse by the fact that the platforms' own search functions are outright braindead.
the sites could implement their own search to make it all work if they wanted to. they just dont. but blaming virtualization is not tit.
The irony is that obsessing over smoothness and frames per second is what causes this in the first place.
Browsers can easily render and scroll through entire wads of content.
The problem is, when the "line" comprising an email in the email list of Outlook Web is in a maze of (literally, just looked it up) about 30 divs deep, performance inevitably goes down the drain.
Computers in the 00s could handle inboxes with thousands of emails just fine and scroll through them. Outlook struggles keeping more than 30 in active memory. That's gotta be a joke.
By the way: the solution is rendering tabular stuff in, well, tables instead of armies of divs that try to be tables. That case has been optimized to death because that is how websites and applications used to be built.
Can't recall the name, but I remember thinking that was very clever once I understood why it was doing that, and am glad to see the concept explained directly here.
Anyways I stopped the project for unrelated reasons and this thought kept nagging me in the back of my mind. The time I work on the project I’ll be sure to try some techniques in the article.
On the price ticket example, on my device (Samsung A15, mediocre phone from 2024) I get 11 fps with the slow example and 30 fps with the fast one. (15 if I scroll!)
Is that a case of the back pressure you mentioned?
All the other (fast) examples are 60 fps though :)
One thing that I found quite interesting to see is how the behaviour of the "4.000 particles" demo changes between the 5ms rendering and the "everything now" rendering.
The streaming response UI/UX is a total nightmare to make work smoothly. You have to come up with clever heuristics around where to chop up the stream of changes and how to batch the work relative to frame updates to make it not look like confetti during a streaming response.
Seems like an artifact of the time when tokens-per-second were low enough that people needed to be informed that the machine was actually doing something.
The problem is that the content is multimodal, often recursively.
You can be inside markdown and then have a python code section. Simply detecting these boundaries is already challenging.
How do you expect me to type if pressing a button moved text focus away from the text field?
But for people who are developing websites that are more than just text, this is an excellently written article and great advice.
It's funny, for mobile app dev it feels like the community are a lot of focused on staying off the main thread, this is the first time I've come across one for webdev.
What's your point? Are you saying all animations and rendering are unnecessary?
> When we run into jank like this as developers, the usual reaction is to wonder “is my code slow?” and start picking apart algorithms or looking for wasted computation. In most cases, though, the speed of the code is not the problem. The code isn’t slow. It just happens to be the code that’s holding the main thread.
Eh, most of the time it is slow, because you chose to use React. React used naively scales very badly. You have to jump through lots of sometimes-fiery hoops to make React… not much slower than some of the alternatives. Memoisation (possibly now via React Compiler) and things like that.
> [Diagram with rAF, Style, Layout and Paint attributed to the main thread]
Paint hasn’t been on the main thread in Firefox since 2017: https://mozillagfx.wordpress.com/2017/12/05/off-main-thread-...
Not sure about other browsers. Firefox has generally been the leader in these areas.
> [steering cost demo]
The demo is deeply unsatisfying because the simulations are quite different in a very problematic way: in “all at once” mode, it loses most mouse movements, which radically affects the simulation which is attuned to movement events rather than simulating physics based on a constant tick rate. Without examining it closely, my guess is it’s using pointer events and the browser is coalescing the events because of the blockage and it’s only taking the final state rather than going through all of getCoalescedEvents(). In a simulation like that, if you can’t run it at full speed, you have to decide what to do, and you should do it deliberately, because it may be disastrously wrong. Should you fall over, simplify the simulation by dropping some of the calculations, reduce the tick rate, something else?
> The demo below is a markdown editor with a long CHANGELOG open. Building the preview means parsing the entire document (about 2,000 lines) and rebuilding its DOM from scratch, which is far too expensive to run on every keystroke.
No it doesn’t. This is a clear example of your code being slow, because you’re doing the wrong thing. You want an incremental parser of some kind. Markdown is tolerably well-suited to this kind of thing because for most edits you can just render the current paragraph.
The suggestion is bad. “Debounce 300ms” behaviour is obviously worse for small documents.
> DOM writes can be batched as well. Appending a hundred nodes in one operation instead of one at a time, or toggling a single class instead of changing style properties individually, turns many changes into one and helps performance. The old technique of assembling an HTML string and assigning it to innerHTML in one shot has the same essence. You gather the writes so the rendering pipeline’s fixed cost is paid once.
This is simplified far beyond accuracy. parent.append(one_hundred_nodes) and one_hundred_nodes.forEach(node => parent.appendChild(node)) are unlikely to perform particularly differently. Changing style properties individually isn’t that* much worse than toggling a single class (it amounts to rewriting the style attribute a bunch of times, and the CSS parser and serialiser aren’t particularly slow). Building an HTML string and assigning it to innerHTML is better than constructing children manually in some cases, worse in others. The real thing that matters is avoiding triggering layout unnecessarily by things like accessing clientWidth between changes.
> Deferring
The lazy rendering trick used here is very problematic because you still need to know how tall each item is. Plenty has been written about hazards of infinite scrolling, that’s what’s being dealt with here. Now if you can have each item be a fixed height, then progressive rendering has far fewer side-effects. I would also say that switching from Notifications back to Feed and having it take an extra few hundred milliseconds is really not that bad. And that I’m not convinced the approach taken was right anyway, you could often just leave the feed rendered and avoid affecting the gross layout.
—⁂—
On the presentation of the site itself: this makes no sense:
.post-content ul {
word-break: break-all;
}Also, the issue I see is who controls browsers as much more profound. We need to find a solution here, as the browser is too important to allow private companies to keep mankind hostage.
Edit: Wow, and the praise-accounts. Is that new on hackernews?
I’ve worked in the field for a long time and see a carefully written, correct, calm article. No obvious AÍ tells in wording or sentence construction. It is a bit long - might have been inspired by ciechanow.ski
I realize the author may not write native English, but this is not translation slop, it's AI writing slop.
It reads pretty much like every well-written technical article used to, ten years ago; and scores a 0% likelihood of being AI generated in Quillbot.
https://github.com/neomjs/neo/blob/dev/learn/benefits/body/O...
Just use multi-threading. Run each library in a separate thread. Use actors approach to pass data between libraries, application and finally to submit data to update GUI.
Using single thread for everything and expecting that developer will create separate threads for heavy work is obvious approach, but it never worked. Developer doesn't care. And user gets inferior experience.