“Why should I use Nyxt if I can use Vimium?”

So after a quick install, I think the problem is that Nyxt is an emacs-friendly browser that's trying to sell itself to Vim-friendly users? Most Vim keybindings are not available (":" for executing commands, "h,j,k,l", Escape, etc...)

Also, the configuration format doesn't make it particularly easy. I don't want to learn LISP so that I can set my proper keybindings. Granted, I'm not fan of the vim language itself but it's pretty easy to set config with it.

In short, the question indicates a category error. Vimium/etc are plugins that make the browser interface a little more keyboard friendly. In a sense, that's the least interesting consequence of having a browser environment that exposes its full programmability.

How many of these things are fundamental limitations of plugins? (Besides the "can start native programs," of course; though I thought one of the Vim-keybinding-plugins had an optional native component that exposed an HTTP interface to allow that.)

I could imagine an extension that's modal and allows its own plugins to be written in JS and then loaded. This doesn't seem at all to be a fundamental limitation, just "Vimium et al aren't designed this way."

Similarly, couldn't an extension use Web Workers to do the intensive processing tasks without blocking rendering, with the only on-main-JS-thread actions being bridging the Web Workers to the DOM?

I guess extensibility is much easier with CLOS methods than with... however people write extensible programs in JavaScript. Prototype hackery? But one could again port CLOS methods to JS as a library, so that's not a fundamental limitation either, IMO. (Maybe the fundamental limitation is the JIT being able to optimize them? I think a normal method cache on the GF might work, though I'm unfamiliar with how JS engines optimize indirect calls in general.)

The reason I use Vimium rather than qutebrowser is simply because the former has the standard compliment of Chromium plugins and the latter does not, or at least last time I tried it.

It's hard to live with many of those plugins. — Though I very much loathe that the plugins do not work on “special pages” which have to be navigated in an inconvenient manner.

I suggest you first introduce the Windows version, then tell us about the differences. What's the point if the majority of users can't see the differences first-hand?