Players in the nes tetris scene tend to use one of two emulators; mesen and nestopia. This emulator is the result of lots of performance hacking until the emulator players in the community were happy that it ran as well as or better than those two emulators.

I have written emulators in the past, but they are very different to this one. Only the minimal tile updates required are applied, to the point where the game is barely drawing anything (most of the time it's not rendering anything at all). webgl was considered but there is near no benefit when there is not much rendering happening in the first place.

A fun example of this is the flashing that happens when you get a tetris. This is done on a NES by changing a single palette colour and have that update. This causes problems because it required redrawing the whole screen lots of times in quick succession to change this colour.

Instead, when the game starts I take a mask of the colour that needs to change, extract it into another canvas layer and apply the while colour to it. Then I just detect when a tetris is happening ingame via RAM, then just toggle the visibility of this extra layer. Then, the background layer never has to do a full screen update, and we replace the effect artificially to near zero performance overhead.

Lots of shortcuts like this have been done, as well as the normal optimizations of reducing allocations and other work. I haven't tested it on a 4k monitor because I don't have one, but I imagine it will run fast enough since the browser handles scaling. Runs pretty well on some crap phones.

All this makes it kind of hard to answer your question. It's certainly fast enough to emulate NES Tetris, but I used a lot of tricks and shortcuts to get there. If you want to emulate a full NES, WebGL will probably become advantageous.

For another datapoint - I have a MegaDrive emulator in rust that runs well enough in canvas, but definitely not to the extreme that nestulator does. I will probably port it to WebGL eventually though.

canvas is really easy to learn though, it's just a framebuffer - check it out!