[Hummingbird] WordPress back end running slowly

Hi there, I've optimised our site using Hummingbird, caching etc. The front end is really fast (which is great) but the compromise is that the back end is quite slow and frequently times out. Do you have any advice on how I can pinpoint what the issue may be and resolve it? I don't want to turn off the caching and optimisation that is in place if I can help it.

Many thanks, Alec