There was recently a bug related to this on the gtk issue tracker too: https://gitlab.gnome.org/GNOME/gtk/-/issues/3714

I don't think there is going to ever be much interest in getting this fixed in GTK3, but apparently performance has improved in GTK4, which should be hardware accelerated on macOS now: https://blogs.gnome.org/chergert/2020/12/15/gtk-4-got-a-new-...