I have a bug(?). In Qubes 4.3, if I open a video in a qube using mpv, everything works fine. But if I then open a second video in the same qube using mpv, the qube immediately freezes (i.e. slows down to the point that I cannot interact with it or even with dom0). Using htop I have confirmed that this is not happening because the qube runs out of memory or cpu power. I did not experience this problem in Qubes 4.2.4. I’m using an AMD Ryzen 9.
I’m just using mpv as an example. The same thing happens if I open any two programs that consume a reasonable amount of memory / cpu. But it doesn’t happen if I open, say, mpv and mousepad at the same time.
Are you sure about this though? When a VM is extremely overloaded the diagnostic tools may no longer be reporting accurately as they too are being strangled. Perhaps you could check journalctl -b in the VM (once things settle, or after a reboot) to see if there is better retrospective info there.
Look at the tips here about improving mpv performance on Qubes:
Also, you can bump up the vcpu value of the Qube so it gets more CPU priority. That could make a big difference in the scenario you describe.