Thanks for your reply Fred, I did receive an explanation from Parallels. I'll copy and post it here.
Hi Sir,
Im disturbing you regarding the performance issue.
As from the symptoms, the issue can lie in NVRAM preferences of your particular Mac.
Please do the following to solve:
open Terminal application on Mac (Applications/Terminal.app)
please type this command (you may copy-paste it to the terminal from this message):
sudo nvram boot-args="debug=0xd4e"
Hit Enter.
You will be prompted to enter Mac user password. Please type it and hit Enter.
Note: you won't see what you are typing, just type and hit Enter. If you don't have Mac user password, please create a temporary one.
Then you may restart your Mac and the issue should be solved.
If not - please reply me here and I'll create a ticket for you in Support.