Do you use a large frame and/or a small
font?

In fact, yes, my screen is 2K, and system font scaling is set to 1.44, but this also happens on regular 1080p screen with regularly sized font.

And how many mouse-wheel events Emacs receives in your
scenario, anyway?

Sorry, how can I check that? I do think that Emacs actually receives a metric ton of events, as scolling with mouse seems hardly lag at all, and scrolling in termux with touch events also not as laggy.

Oh, and what happens if you raise gc-cons-threshold to a large value?

No difference. Note, that scrolling with scrollbars has no lag at all, so I don't think that GC is relevant here.

I've also noticed that scrollin with scrollbar has (sit-for 0) in the body of the scroll function, and IIRC mwheel-scroll doesnt sit at all. maybe because of that it consumes too many inputs?

I wonder if you could send tons of scrolling inputs via xdotool or smth like that on Linux?