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.

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vidarh an hour ago | parent [-]

MUI was slow because it layered on a crazy amount of configurability. You didn't need things to get slow with resizeable/dynamic layouts even then.

	▲	amiga386 4 minutes ago \| parent [-]
		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.