Sorry it took a while to respond...
The slowdown for me at least has always been a problem, for as long as I have used Test Complete. We are currently using TC 8.7, and it is stable enough that I don't dare venture forward. I have used TC9, and 9.1 in the past. I did not try TC10, and I couldn't anyways since there is no way to convert my TC8.7 scripts without having to do loads of changes (and in a ~4500 tests project, that is just not realistic). There are severe memory leaks and performance issues in TestComplete, at least for all of the versions I have used. Some are better than others, but the bottomline is if it doesn't crash you should probably consider yourself happy.
Others and I have pointed out many performance issues that still remain unresolved to this day. Smartbear is more interested in adding features and bloat than solving these issues in the long run it seems. It's probably just easier.
In any case, on VMs there are TONS of things that can be misconfigured without you even knowing about them. Having the slowdown issue on a physical host however is weird. Two identical machines started at the same time should pretty much take the same amount of time to complete the same tests. If one is significantly slower than the other, but proper benchmarks see no difference in performance, there is a good chance some software or service is interfering, and often that is an anti-virus or something similar. As a rule of thumb, your test machine or VM should be bare bones. That means disable every non-essential service (like Windows Search, or System Restore, Compatibility Assistant, Windows Firewall, etc...), clear up any non-essential third party sofware, including anti-virus or other resident program, and make sure there are no connectivity issues. If after all that there is still a large difference in performance between the two machines, it might be a BIOS setting (especially power management like SpeedStep... also pay attention to Intel VT options in there...).
You can expect TestComplete to be slow and sluggish after a while. That is why in my case I kill the process completely after each test and start it fresh.
