Done a little playing, and you can get a fair turn of speed if you use IceWM, wmaker or Blackbox as the WM for Monodevelop.
My criteria were
1) no menu fiddling. If the WM didn't pick up LXDE .desktop files it was game over
2) Acceptable speed
3) Acceptable stability.
NOTE - 2 and 3 are obviously subjective, so other users' mileage may vary on how well I've selected.
I started from this post http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=7571
and then winged it from the f1 alternate sessions listed in SLiM (I know, SLiM is for the weak, but idleness is nothing if not worked at... if you want the CLI login press Ctrl-Alt F<pick an number between 1 and 6> and you will receive
For completeness, I tested
LXDE - geologically slow, and could crash the RPi completely. Could have been a long time out, but the clock wasn't updating, and after 15 mins I called it 'crashed'.
XFCE - a little faster than LXDE but also crashed the whole machine, more or less the same as LXDE
Openbox - it may run but needs to be messed with for menus to reflect what's installed.The rest seems sprightly. I didn't look further.
IceWM - closest to LXDE in look and feel, but it works fast, when it works. At least only the application crashed rather than it taking out the RPi, and after a couple of reloads, it didn't happen again.
WMaker - works in a usable fashion, but is a little ugly IMO.
Blackbox - my personal favourite, but not LXDE look and feel.
Awesome - it looked usable enough, but didn't pick up the Programming applications and the text seemed to be the wrong size for the menus. GUI seems responsive. I didn't fiddle further.Conclusion.
I had a couple of crashes in IceWM, I didn't like the feel of wmaker, and so have decided to go to Blackbox, at least for the moment.