Host Mageia 9 on AMD Ryzen 5 3600XT 6-Core Processor running Plasma X11 on an SSD equipped system.
VBox configured to use 2 cores (4 CPU) and 10 Gb out of 16 on this system and the virtual disks on the SSD. This applies to all my VM's, and no problems with Mint or Fedora or EndeavourOS VM's. I only run one at the time.
But when I start the Win 11 VM, it seems to monopolize the whole system for a few seconds e.g. playback of a CD stops for a second or so, and that 3 - 4 times during the startup of the Win 11 guest.
Once started, the system and the VM perform OK. But when I shutdown Win 11 and the VBOX, my host desktop gets sluggish for some 10 seconds, then the speed improves a little, without ever returning to normal. And the worst effect is that - half an hour or more after shutting down VBox - shutting the system completely down takes some 30 seconds, where normally this takes some 5 seconds.