Hacker News Reader

Comment by danpalmer | Hacker News Reader

> Humans can’t notice 100ms or 200ms latency

My gut is that there's a lot more to this, and that it's not broadly true.

I live in the UK and have worked from Australia a few times, and the latency is around 220ms. When typing over SSH, the latency is very noticeable. Not in terms of pressing a key and feeling lag in it showing up, but in a feeling of sluggishness when typing more than a few words at a time. I also notice this feeling in web apps that use React managed inputs for example.

My guess is that while 100-200ms isn't noticeable in one instance, adding that to every interaction in a system that has multiple interactions per second is noticeable.

A similar comparison is that I can't detect audio and visuals being incorrectly synced with much less than 200ms precision for individual beeps, however if I watch someone talking on a TV that's 30ms out I can tell something is wrong (even if I can't tell which direction).

View on HN

If you read a little closer this is in his "counter-arguments" section stated as a claim made by other people - a claim he doesn't buy. If you look at some of his other articles about latency he points to user studies and research that shows the 100-200ms claim is not true.

So he's making the opposite argument you think he's making - he agrees with you.

To me it seems that parent comment is intentionally agreeing with the author, and just adding his own perception (which corroborates to the point).

The rule of thumb floating around when I was working on physical input devices last was to keep latency under 50ms so people wouldn't get frustrated. I don't have any sources but it becomes a bit more obvious when you look at it as fractions of seconds and then compare that to how slowly seconds actually tick by on a clock.

I remember Dan posting that he typed at ~120wpm somewhere. 120wpm is 0.002 words per ms or 0.4 words per 200ms. Wouldn't one notice if they're almost halfway through a word and nothing has shown up on the screen?

> Wouldn't one notice if they're almost halfway through a word and nothing has shown up on the screen?

There is quite a delay between the eye detecting something and it being consciously processed to the point of being used in decision making. Away from built-in reactions (move hand away from burning sensation and so forth) that are not consciously controlled there is a sometimes surprising amount of latency in our view of the world. It is surprising because the brain does a rather good job of smoothing it out with guesses/predictions such that you generally don't notice.

Only when things fall outside the range of patterns you naturally cope with or expect because they are consequences of your own actions, do you really become aware of them. So he might not notice usually, but would if an error or something else unexpected occurred.

This is why 100ms, or even sometimes 200ms, is fine for many things, but others cause irritation despite delays of 40ms or sometimes significantly lower. The brain smooths out what it can, the rest sets of warnings of potential trouble.

Not sure why this is being downvoted.

Even for certain applications, 50ms is quite a lot. When using MIDI devices and playing piano over the computer, I certainly notice anything above 20ms.

We may be slow to react to sudden events, but we are very good at noticing the lag for predictive events (keyboard typing, piano playing, ...).

Yes, a single 60-Hz-frame delay (~16.7 ms) can certainly be noticeable.

Martin Molin of the Wintergatan fame recently posted some experiments regarding latency on his YouTube channel[0]. From watching those videos it becomes quite obvious that your statement is definitely correct.

[0] This is the first one about the recent experiments: https://www.youtube.com/watch?v=VyIk0IqC7SQ

Yeah this rule of thumb was for low fidelity things like hand radio buttons.

> We may be slow to react to sudden events, but we are very good at noticing the lag for predictive events (keyboard typing, piano playing, ...).

It does make sense in my lay person’s understanding that the chain of eye -> brain -> hand for reactive events would be slower than brain —> eye + hand for proactive events.

I work on VR systems, where latency is of paramount concern.

There is definitely a big difference between consciously noticeable latency and latency that has an effect on people.

Yes, to get to the point where a person can notice that latency is occurring and can call it out, you need to be in the 100ms range.

But if you want to make a VR system that doesn't make most people physically ill, you have to get "motion-to-photon" latency down to around 13ms (75hz). Even at that range, you still get a relatively large portion of people experiencing issues; I'd estimate 20%. At 90hz, you can get that down to about 10%. At 120hz (less than 9ms!), I haven't seen specific numbers, but my gut feeling from my own work with users is that it's lower enough to care about the extra work to hit that target. I suspect that we can't be fast enough on this issue, that 240hz, maybe even 1000hz, will be necessary to get down to 1%.

Luckily, most VR systems today have asynchronous reprojection systems to decouple view movements from application updates (to a degree). It is an extrapolation system, so there are some artifacts, especially around the edges of objects, especially if there is a huge difference between application update rate and view update rate. But I've seen applications running updates at 30fps, which should be headache inducing for sure for the vast majority of people, but they're running on headsets with a 120hz refresh rate and all you really notice are that animated objects look a little choppy.

Oof. I play piano (as a hobby, but seriously). When practicing on an electric piano, anything more than 20ms of latency is VERY noticeable for me, possibly even 10ms. There's a reason people still use wired headphones for practicing on electric pianos. Bluetooth just does not work. The latency interrupts the emotional state of flow and it's extremely jarring.

Typing on a computer is a different story. The latency perception via the eyes is very different than through the ears.

I play guitar, but I'm in the same boat (probably with a longer signal chain) -- 10ms is about the limit before it's noticeable. The idea that people don't notice 100ms is crazy talk; anyone playing 16th notes at a pretty reasonable 120bpm is hitting a note every 125ms.

Our brains are much more sensitive to phase/delay in audio sources than any other stimuli. There is a reason the lip sync setting on many home theater setups can be changed in increments of 1ms or better.

I play a lot of video games. In Tekken e.g. Heihachi has a move called OTGF. The input sequence is listed as f,N,df+2. Each of those movements must be done in 16.6ms or it doesn't work. On an 8-way joystick, there are only 4 switches for the cardinal directions. That means the final movement of df+2 requires hitting the d switch, f switch, and 2 button within the same 16.6ms window to count as simultaneous.

There are players that can do OTGF move every time they try.

My audio interface for connecting my keyboard/guitar to my PC is USB-C. I tried USB2 devices and find they have too much latency. It's very noticeable. USB-C has 1ms latency or less.

It's a world of difference playing FPS shooters online between a 50ms ping and a 100ms ping. If you care about your aim, 100ms ping is barely playable.

I remember playing CS at the turn of the century on French or UK servers with 100ms or a bit more without problems, could it be that the problem is that now we are sending more stuff than before?

I have friends who still play (making it 20 years). They're at a level where playing on public servers gets them banned for cheating (some are/have been semi-professional).

Any latency, whether it's hardware, software or server, drops their performance significantly. It's at a point where they only play/practice under prime conditions, as their sensitivity is such that they will throw themselves off by practicing with more latency than they normally do.

It's crazy.

In FPSes in particular, every millisecond of latency typically translates into peeker's advantage due to how lag prediction works. Or sometimes disadvantage, in the netcode that is poorly implemented or optimized for poor connections. It also depends on the movement velocity.

In games with really bad netcode it can get ridiculous, influencing the entire gameplay - for example in PUBG beta, which had about 200-300ms of actual client-to-client latency due to the very low server tickrate, you had to avoid running directly towards the corner to look around it, as enemies could see you running out in the open for a split second then rubberbanding back.

CS (HL) lag prediction is a joke, you could sometimes see how an enemy 'leans back' to catch that headshot.

Play Zandronum on 150ms server against people with 30ms. You will go for a new mouse (or maybe even a keyboard and monitor) in half an hour.

>There are players that can do OTGF move every time they try.

That has nothing to do with latency. Human nerve conduction velocities just aren't quick enough for things like that to be done closed-loop under the control of the brain - it's just physically impossible.

Although the person doing the action may feel otherwise, that's just a well-trained sequence of actions performed open-loop.

> My audio interface for connecting my keyboard/guitar to my PC is USB-C. I tried USB2 devices and find they have too much latency. It's very noticeable. USB-C has 1ms latency or less.

Are you sure that's not just USB 2.0 interface being shit ? USB2.0 is certainly fast enough to have sub-ms latency.

I actually wondered about how to go with testing USB interface latency, maybe I should test mine when I have slow weekend...

One place where latency has been felt very obviously for me is the MMO World of Warcraft. I’ve played it for many years from several different geographical locations, with reported latency between my computer and their servers ranging from as low as 20ms to as high as 240.

I can pretty reliably tell if I’m playing on a PST or EST (North America) server, with the latter having 50-60ms latency compared to a PST server with 20-30ms due to differences in distance. It’s not massive, but on EST servers everything is ever so slightly less responsive, and it’s particularly visible in combat where many abilities are being used rapidly and outcome is dependent on order of operations.

The differences starts becoming obvious when crossing the 80-90ms latency and unmistakable at 120ms+.

I would agree that it would be difficult to tell the difference in a one-off test but when it’s something you’re actively doing for long stretches of time it’s easy to pick up on.

I played WoW on dialup will into the lifespan of WotLK. Average ping in the 320ms range. I learned to play around it to a degree, 'queueing' my spell casts by triggering it when my current one was close to completion, such that the command would hit the server about the time it would complete my previous spell. I could reliably chain cast spells this way once I got used to it, using the order of operations to my advantage as best as possible, as you say.

Once I moved and got onto broadband, and found myself with a miraculous 100ms ping, I basically had to learn the game all over again as my instincts would cause me to try to do actions I couldn't yet do, since I wasn't quite done casting a spell, harvesting a node, etc.

Later on I changed servers and was below 80ms ping, and the difference was again palpable but not game changing.

I also notice this feeling in web apps that use React managed inputs for example.

I can tell when there's an extra render in a React app, on an input or just when something renders slightly later than expected like an update is being done in a useEffect after the first render, and I agree that it's really annoying. However, the input latency of a React app should be less than 16ms. There's nothing intrinsic to React (or any JS framework) that means there has to be any appreciable lag. Obviously there are badly written ones that do things very poorly, but if you find one like that you should vote with your feet and move to a competitor.

> Obviously there are badly written ones that do things very poorly, but if you find one like that you should vote with your feet and move to a competitor.

What if there are no well written ones? No competitor to move to? I for one haven't seen a single React app that wasn't annoyingly slow, under the standards we're discussing here.

> There's nothing intrinsic to React (or any JS framework) that means there has to be any appreciable lag.

And yet, for common SPA frameworks, it seems it's always there. So if it's not the framework and the language, then something common in the overall process of frontend dev is a likely candidate.

What if there are no well written ones? No competitor to move to? I for one haven't seen a single React app that wasn't annoyingly slow, under the standards we're discussing here.

If that's the case then React is a huge problem.

I've seen lots of apps that nail 60fps that are written in React though, so I don't think it is the case.

I'm in New Zealand have a VPS in Eastern Australia, so a little over 2100km (1600mi).

Average ping is 63 ms. Even that latency is noticeable - more typing errors for sure. I never connect without mosh.

mosh is a godsend and I wish it was more actively developed

What is it missing that needs working on?

Iirc the thing I miss the most is effective ssh key forwarding.

Have you tried Eternal Terminal?

https://eternalterminal.dev/

This looks interesting, I'll have to try it out.

People can definitely notice not only sub-100ms, but even sub-10ms.

Case in point...

My previous iPhone had 60Hz refresh rate, that equates to ~16ms.

My new iPhone has 120Hz refresh rate, that equates to just ~8ms.

I can absolutely tell the difference in the display between these two devices when I scroll.

That does not show you can distinguish < 20 ms latency. Latency is specifically the time between you taking an action and getting a result. I would doubt that most people would be able to tell the difference between 8ms and 16ms of latency between a button and light or speaker.

There are two other ways scrolling on a 60Hz vs 120Hz phone would allow you to tell the difference: the increased smoothness of the scroll (not related to latency at all) and how far the content lags behind your finger. The content lagging behind where you initially touched does allow you to experience the latency, but that is more of a visual only experience than a temporal one.

> The content lagging behind where you initially touched does allow you to experience the latency

Apple reduced the latency in its pencil from 20ms to 9ms. It's well regarded that the reduction of 11ms in latency is very much noticeable.

https://news.ycombinator.com/item?id=20091610

In case of this particular interaction its just easy to notice because you have direct view of pen position and what it is drawing so any latency means the pen is dragging but there is nothing under it.

It's different for where you don't have same senses for both. "Pressing" a key is feeling + sound but reaction is change in image, you don't see keyboard and screen at same time so it feels more tolerable.

> "Pressing" a key is feeling + sound but reaction is change in image, you don't see keyboard and screen at same time so it feels more tolerable.

It's not really more tolerable, unless you never experienced low latency here, and your brain is used to the effect of a key press being perceptually immediate.

The author is rebutting this. "This line of argument is like saying that you wouldn’t notice a flight being delayed by an hour because the duration of the flight is six hours."

I was messing around with CSS animations the other day and even at 100ms you can definitely see the animation happening. One of these days I'll get around to making a site to A/B test how fast they are noticeable.

Maybe there are circumstances where 100ms+ latency is not noticeable, but I suspect they are few and far between if you're actually paying attention.

> even at 100ms you can definitely see the animation happening

See it happening is very different from noticing lag. Here's a gif I like to reference when trying to determine how noticeable a lag spike is in milliseconds. For me it become unnoticeable at a 30ms lag spike. But 100ms is very plainly obvious.

https://images.app.goo.gl/95v3yNyj3fdm1o1v6

Edit: Also, movies are typically filmed at 24FPS which is around 41ms per frame. There's something called the "Soap Opera Effect" which is when you're watching an older movie on a newer TV that automatically interpolates frames and boosts it to 120FPS (8.3ms per frame). People have noticed this effect so much that it has its own term coined after it, the soap opera effect. That's a 32ms difference, and people may not immediately be able to tell what's wrong, but they know something is off

https://www.techtarget.com/whatis/definition/soap-opera-effe...

For me it's noticeable down to around 20ms, although of course it becomes less noticeable the lower you go. Surely the existence of high FPS screens is clear evidence that humans can see latency spikes? 60 FPS is ~16ms. 120 FPS is ~8ms.

> https://images.app.goo.gl/95v3yNyj3fdm1o1v6

Interesting. I still notice it at 20 ms, but not at 10 ms.

>> Humans can’t notice 100ms or 200ms latency

>My gut is that there's a lot more to this, and that it's not broadly true.

Which is why that quote is from list of "counter-arguments people say" and debunked by author...

> I live in the UK and have worked from Australia a few times, and the latency is around 220ms. When typing over SSH, the latency is very noticeable.

You don't have 220ms latency. You have latency of your whole system plus that extra 220ms

This has been debunked since 2014, humans apparently can detect meanings in pictures flashed in front of them in as little as 13ms.

https://www.researchgate.net/publication/259490307_Detecting...

> Humans can’t notice 100ms or 200ms latency

Yea, author never played MMOS or FPS games online?

Going from 20ms to 100 is going to be VERY noticable.

Games are a bit different – how the latency is measured depends on the game, and typically doesn't account for the rendering or local state update required to render correct data, may not account for the server's computation of the new state, won't account for input latency, etc. I agree that it's very noticeable in games, but a quoted 20ms network latency on a game could probably be anywhere from 40 to 200ms of effective latency, so I don't think they're a great example to use here.

What absolutely everyone is measuring today in games is motion to photon latency, with the help of a high-speed camera. This is the only thing that actually matters, along with motion-to-photon over network (with RTT measured separately), so you can tell how well both client-side and server-side parts are optimized, how high the server tick rate is etc. For the server part, different actions such as movement or attacking could even have wildly different latencies, depending on how it's implemented in the code.

The most responsive games achieve sub 30 ms motion to photon locally, with proper hardware.

The linked comment is poorly written — that is actually the claim which the author of that comment is arguing _against_. He has the correct answer which is that you need to be somewhere around 10ms to be imperceptible but explains where some of the different claims are coming from.

There are a number of confounds for measuring things like this because we're often not talking about exactly the same thing: for example, the times needed to perceive motion, size, color, orientation, and recognize something are all different (to a first approximation, the relative latencies are roughly what you'd expect if the most important selection criteria was a relatively weak fructivorous ape trying not to get eaten) and there are measurable differences between how you respond to things you're predicting will happen, continuous motion, or truly unexpected events. This also varies considerably for novel and trained behavior — e.g. if you're waiting to shoot when you see a pixel change, your body is primed and presumably quite well practiced vs. something random happening in a video. All of that means that something like extra keyboard latency is especially noticeable because you're initiating the action, it's extremely practiced, and you thus have a well-honed expectation for when the response should be visible

mosh is supposed to be pretty good for ssh latency: https://mosh.org

Reply on news.ycombinator.com