The GUI was fast because it was fixed layout, MUI on the Amiga was slow as well. We tend to go that way to get UI that is easy to create. You see the same in all GUI libraries the more CPU you have the more you use it.
MUI was slow because it was written in C and went all-in on BOOPSI, as well as supporting fully dynamic layout. It could likely have been faster, but the sort of people who paid for it were also the sort of people who bought CPU accelerators.