OS: Arch Linux version: 1.30.1 gtk3 from Arch's AUR
With GTK3 on Wayland, vertical scrolling is laggy. When scrolling slowly, its fine but as soon as I scroll faster (possibly one line per frame is the threshold) it starts lagging. It immediately comes to the correct position when I stop scrolling, and occasionally updates while I'm scrolling (about 1fps). Everything works if the window is smaller then half the screen (screen resolution is 2560x1600), but there is still high CPU usage while scrolling.
I encounter the problem on Sway and Gnome on Wayland, but there is no issue on XFCE4 with X11.
Based on the high CPU usage and dependence on window size, my guess is that it is CPU rendering and, if more scroll events have come in before it is done rendering, it renders the next frame and keeps going until it has caught up.