Back-end Performance Issues

Ever since I've moved my four separate wordpress installations into one MU installation, I am getting lots and lots of issues with very slow response and time-outs on the backend. I am hosting with tsohost in the UK using their grid server. The front end is super fast- no issues whatsoever. But when I get to the dashboard and try to do anything like add a post, edit a post etc, usually one out of every 3-4 clicks results in a timeout.

What could be the issue?
Would appreciate any help.