I'm running two externals on a 27". I know you're asking about the 21", but pretty much the same thing. One is a 9-year old 23" cinema display using DVI with a thunderbolt adapter (i think I have thunderbolt on this 27"). The other is a 5-year-old cheap-ass 23" monitor also going thunderbolt>DVI. 27" is running at full 2560x1440, cinema is running at 1920x1200, and the cheap monitor is running at 1920x1080. It works perfectly for me...no hiccups.