I’ve noticed a few times that my Ubuntu VM, running in Parallels, suddenly goes slow.
First time round I was totally confused and re-booted the VM (I access it via SSH), that didn’t work, and was also really slow. So I shut it down and then I noticed when I returned to Parallels to stop the VM that a dialog box was waiting for me (telling me there was an update or something). It turned out that this was the issue. It seems to happen to me for any dialog box as just now I’ve had the same issue when there is a dialog box asking if I want the VM to connect to a USB device.
Seems silly to me that this should slow down the VM, but just thought I’d mention it.