My suggestion is putting on so many at a time and taking note of the ones put on. Then if it slows down, you can remove the ePub put on and try different ones. Maybe there's a problem with some of them.
You could run them all past epubcheck 3.0.1 to make sure they have no technical errors that could cause a problem.
My suggestion is ePub-Checker.
http://download.pagina-online.de/epu..._Win-1.2.0.zip <--- Windows version