Taronites

Ah, THANKS for uploading the video, very helpful!
Looks like you should update your Firefox, possibly? This is what it might look like, if you only had webGL1 available as opposed to webGL2.0.
In my Firefox (both 32bit and 64bit) here it totally won't have such troubles and looks perfectly smooth without faulty brushes like that. This happens, if velocity won't get stored and used to create streaks from stipple to stipple.
Again, thanks for showing me that and that's very curious. Let me know, if updating has helped?!

Hi, Taron
in my firefox 89.0.2 the stroke is a bit jumpy, I mean the first 1200 ms or so of every stroke of my stylus never appear in the screen, but the rest of the stroke goes almost fine.
Hope this helps somehow.

my current firefox is 90.0.2 and 64bit ...
I did made a test with a clean, noew profile, without add ons, hmm, Opera still much better results.

By the way, this sandbox tool, is very cool

That looks very nice, Zero!
Yeah, it's weird with the browsers and machine setups. I'd have figured, it would be a tiny bit more predictable, but alas...
I have noticed a strange behavior from time to time with the Wacom, when very quick, small strokes can lead to a sort of hanging start location, where it jumps to the start position of the previous stroke. I could not find any reason in the code for this to happen, yet. Logically it shouldn't be possible, because mouse down resets the path, clears all vectors and places the brush cursor exactly where the mouse got down. But it feels as if somehow in the browser or tablet drivers a kind of buffer doesn't act right all the time. ...I still have to investigate further. ...also, this does not happen all the time. It's as if something could gradually fall "out of sync" in a way?!

Thanks, Ed. 1.2 seconds of hang time???? ...that would make it virtually useless! I know that when one first starts it, this can happen, but it catches itself immediately and then it works fluently except for the above issue, of course.
Hmmm...I should record a little video, I guess.

https://www.youtube.com/watch?v=V2BDHs2OAB8
Ok, I've recorded a quick video with Firefox 64bit on Win7. Recording has slowed it down insanely, but it still worked just fine without any stroke issues, actually.
Seems like my Sandbox has lesser issues than the version I'm working on right now. But, yeah, recording isn't flattering. Without it, performance is beautiful, actually.

way more jumpy than that Taron
like four times the lag bef the stroke appears on the screen, always when drawing, just drawing, no big strokes.

That's spooky.
AH, your graphics card is a laptop card from 2012. It's got only 2/3rds the frequency/speed of my GTX 970, which is from late 2014 and for desktop. So that already would explain greater challenges. But still... and then there's Windows 8 (hopefully last update?)... which was no good OS with loads of problems.
But all of that does explain a lot, actually. I never quite researched your system specs. Pretty interesting. But it is still so powerful. The real mess are OS and other aspects of browser coding, which simply does not priorities graphics performances on the fringes of "need", like a full-on paint application inside the browser. Thus, I understand that one must not rely on a browser to provide a perfect runtime for any serious application. But it's a pity and hopefully this will improve, because it absolutely can/could.

Ehehehe, a mummified dino?!