Salvage-Mod on the Evo 4G (basically a completely vanilla GB ROM) is very fast. Definitely faster than many of the Froyo ROMs I've tried. I just think it depends on who is creating the specific ROM and what kind of changes, if any, they are making. In the case of CM7, there are quite a few modifications from the original ROM that could be causing the slow performance. That said, there some things from GB that I can't wait to have in MIUI, particularly around text entry (copy/paste, cursor positioning, etc.).