1) When you say slow response, what is slow exactly? Is the mouse cursor moving with a lag inside the VM, or do you perceive a lag when you click on a mouse button and an action takes place in the VM?
2) Which operating system are you running inside the VM?
3) Which version of Mac OS are you running, on which Mac model (run "sysctl hw.model" in Terminal)?
4) Can you confirm that this is a regression from Fusion 4.1.4 to Fusion 5.0.2?. You can install both versions at the same time on your Mac, by putting them in different locations. Then, you can run them one at a time (i.e. open Fusion 4.1.4, use it, quit it, then open Fusion 5.0.2, use it, quit it).