Well ...
I dont say Magic sux :)
I dont say Magic is slow :)
For old Ataris without much RAM it is far better option as decent MiNT setup. Atari comunity need commercial OS as Magic is.
From what you said about your MiNT experiences I say that you dont have optimal MiNT setup (neither do I) and that it should behave much better.
BTW. yesterday I started XStella (Atari 2600, XL emulator for Xwindow) under GEM and it works (with gemx for MiNT). Its slow but on 060 Milan should be more useable.