Magic Workbench is just a new set of icons. I used to have it on my 020/2mb machine and haven't experienced any slowdowns It needs 8 (16?) colors so it'll get as slow as a normal workbench screen with that amount of colors. While MWB icons are not perfect, they are better than default ones.
Twist, maybe you're speaking of MUI? I remember no help windows with MagicWB.