Scrolling with QtWebEngine seems to be very choppy on some pages, possibly with large background images involved. QTBUG-90543 was one possible example on this on Qt 5, but ended up being not reproducible, as the website changed just days after I had opened the bug.

https://www.protondb.com/explore is probably the most common example where this happens - it's more visible the bigger the window (e.g. simplebrowser, Falkon or qutebrowser) is. If you can, try it maximized on a 4K screen.

Disabling the background image in css fixes the issue as does disabling "background-blend-mode: luminosity;".

Here is a video made by the upstream reporter, which tests this in qutebrowser, Firefox, Falkon, Chromium, and then qutebrowser/Firefox again: https://www.youtube.com/watch?v=Amh7ntEl9QQ

It is slightly laggy in Chromium (compared to Firefox) as well, but QtWebEngine seems to be much worse.