MUI? Magic User Interface?? First time I heard of that (early 2000's), I thought WTF? A program that sorta/kinda replaces Amiga GUI/Intuition or whatever - that seemingly does nothing different, takes forever to load and utilizes a squished screenmode. Great. I still don't understand why Trident *needs* this program in order to run - but I'm sure there's a good explanation - there always is. lol
You had not hear about MUI until 2000? Were you hidding behind a rock? And what programs did you use in 2000 to connect to internet? Almost all Amiga internet software uses and used MUI. Didn't you use Miami, YAM, IBrowse, AmIRC, Amster (napster)...?
I guess you would also complain when a program required bgui, gtlayout, classact or any 3rd party library.
MUI is not a "program", it's a set of libraries and classes that allow Amiga coders to produce decent GUIs, something very difficult with crappy gadtools or other primitive graphic toolkits.
Have you ever tried out MUI examples? You have all kind of different classes, you have drag'n'drop everywhere and it's a pleasure from a programmer user point. You can set per-application preferences depending on the preferences you want
IIRC MUI3.8 is even included with AmigaOS3.9 as contribution.
BTW, MUI is not slow, try to build similar complex interfaces with Reaction and you will notice it. People who claim MUI is slow usually fill every gui element with textures and later complain about slowness.