Evidence is super easy to come by, but the question does need to be specific and well formed (what kind of rendering, exactly, are we comparing, how many pixels, what’s the bottleneck, etc.). There are loads and loads of benchmarks demonstrating GPUs are faster than CPUs at turning triangles into pixels. Not just a little faster, the numbers are usually in the ~100x range. There’s literally zero contention on this point, nobody is questioning whether simple rendering cases might be faster. Nobody is playing Fortnite with CPU rendering. Because this question is so well settled, GPU rendering is so ubiquitous that it’s even hard to test CPU software rendering.

There are certain kinds of rendering and corner cases where CPUs can have an edge, but those happen in areas like scientific visualization or high end VFX, they don’t come up often in text editor rendering engines.

You can’t use your anecdote of 1 app that might have a broken implementation to question GPUs categorically. I’ve never noticed iTerm2 using significant CPU. My iTerm2 sits at 0.0% CPU while idle. Maybe your install is busted?