Thanks for the hint. I like 8 even better.
3840x2160 with 150% Scaling, Firefox on Wayland (KDE) on Arch Linux.
Somehow, the mouse is not just slow but has a hard downtrend. After swiping 5 times left to right and back again. I look at my feet. The downtrend increases with higher mouse sensitivity.
Maybe it is something to a high resolution screen, so one CSS pixel is actually quite large?