I'm having the same problem with Windows 10 and Ubuntu 20.04. The VM starts off super quick then gets slower and slower until it completely freezes.
The only way I've found to bring it back to life is to suspend it and then un-suspend it... But I have to do that every 10 minutes or so which is a nightmare. If it sits there long enough without me suspending it then it will pop up the message "Failed to suspend virtual machine".
I tried setting the VM's BIOS to UEFI but it fails to boot so that isn't a working solution either 