They made GtkWidget lightweight and fast. So CellRenderers have no reason to exist any more. Then they added new view widgets that use regular widgets instead of cells.
An application can consistently be made using the same primitives that are flexible, themable, performant, etc.
Computers are way faster than they were 18 years ago, so what I want to know is if, in 2003-2004, the code which provided the same functionality was less performant than it is now, or if somewhere along the GNOME development path it became bloated. Because otherwise my mind is blown about how fast my already snappy GNOME 2 Linux desktop could have been.
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.
An application can consistently be made using the same primitives that are flexible, themable, performant, etc.
It is actually really nice.