I think it hard to learn than egui (because there are no official docs yet), but because I'm new to Rust maybe it take long time for me but not for you.

In term of performance, I think both are almost same, both use immediate mode, render by GPU.

The best thing of GPUI is it use tailwind convention for create element like this:

It easier for me (mostly have web-tech skills) to adapt, it feel comfortable with me to work with.