[vidarh]

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]

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.