Michal,
I've been wrestling with the same problem for the last couple of months VMWare virtual machines running distributed tests under Windows 7. One machine always runs about 3 times slower than the other two. It's interesting since, they're all clones of the slow machine. Rearranging jobs among the machines does not change the performance. Lately, a second VM has slowed down also. Restarting the the VM's seems to have some positve effect, as does restarting TestExecute and the application under test.
Suspecting that the cause could be a bad VM build, the machines were rebuilt and reconfigured with more physical processors and memory. This helped, but the difference still remains.
Let me know what you find out.
