Computers don't work like they did 18 years ago.
Today you are rendering to a 4k display (8 million pixels) and every consumer device has a general purpose GPU that is really high performance at specific rendering tasks.
So in many cases, certainly not all, GTK 2 code using Cairo to render will perform worse than GTK 4 using OpenGL even when implementing the same features. Admittedly GTK 4 does use this overhead to do "fancier" things like animations and CSS but these aren't super complex tasks on a GPU.
In my personal experience GTK 4 feels very fast and snappy, clearly more so than GTK 3 was. However I don't use GTK 2 applications to compare.