I don't believe it's a fundamental GTK problem. When you scroll, the Wayland compositor sends scroll events to the app, with a float for the delta. KWin and Sway have an option to let you multiply that delta with a constant; multiply it by 0.5 and you have halved your scroll speed. GNOME doesn't have such an option.
GTK may have other scroll issues as well (I recall seeing some stuff about that, though I don't remember the details), but there is definitely stuff the compositor can do.
> When you scroll, the Wayland compositor sends scroll events to the app, with a float for the delta
GTK works through a bunch of heuristics and in-built multipliers, but the fundamental problem is that scroll effect depends on the “weight” of the content itself — kinematic scrolling is unfortunately quite missing from linux desktop, AFAIK. But do correct me if you know better, I’m not too well versed in that.
GTK may have other scroll issues as well (I recall seeing some stuff about that, though I don't remember the details), but there is definitely stuff the compositor can do.