Here's site that can serve as a (reasonably light) benchmark: https://tiki.org This video shows how EpiphanyDevel "technology preview" (flatpak version from gnome-nightly), shown on the left, struggles to scroll smoothly and to follow the mousewheel's events (or the mouse cursor's drag on the scrollbar), whereas Firefox (shown on the right) has no problem and remains butter-smooth: https://youtu.be/O6ixlqzSDwA The difference is more subtle than bug #215045 or bug #221738, but the jankiness can still be felt.
This can no longer be observed in Gnome Web 44.2/ WebKitGTK 2.40.1. Perhaps fixed already?
It might be worth retesting now with Skia
Created attachment 472817 [details] Sysprof 47 capture on HiDPI screen It's better, though the problem is still noticeable (i.e. not 60-fps-smooth) on a 4K HiDPI display with Intel Kabylake graphics on Fedora 41. Here's a sysprof 47 recording launching this command: /usr/libexec/webkitgtk-6.0/MiniBrowser --private https://tiki.org ...and scrolling with the touchpad.
Created attachment 472818 [details] Sysprof 47 capture - flamegraph screenshot
Created attachment 472819 [details] Sysprof 47 capture - WebKit marks screenshot These marks seem saturated solid, I don't know if that is suspicious…