ensete
i disagree with guests being able to cause strain in a safe host, they never have for me.
but i kinda see what you're trying to get at here -- in some cases, app-dependent strain can happen with gpu drivers enabled. have you running virtualbox linux with gpu drivers entirely disabled even on the host? i.e. Basic Display Adapter on the Windows host
that's how I run all my VMs on my "reliable" computers, both GPU acceleration is disabled for the guest VM but also the host's Intel HD drivers are entirely disabled too, falling back to Basic Display.
performance is still good enough for me on a modern Intel laptop for Linux software dev workflows, I can still have many browser tabs open and videos playing in the VM guest even with Basic Display on the Win10 host.
with Basic Display, IMO this is always able to prevent all "app-dependent" strain as long as the main Windows install is already known to be comfortable for me.