You know, you could just open many instances of a small app from the command line.

Example:

That will open 70 instances of leafpad. I had close to 30 open and the slowdown was less than any one app running in a Xephyr window. Did you know that once you open a certain number of apps, you can use kinetic scrolling in the task manager? So unfortunately you can't see all of them at once

EDIT: LOL tried opening 70 at once and got a bunch of error messages, only about 39 opened successfully. Opened one more and the desktop crashed It would probably be OK if you opened them one at a time.