Ubuntu / Debian VMs Have Performance Degrade Until They Become Unusable

I run two or three VMs usually, and I always have the same problem. Performance starts out great, but slows over time until the VM becomes completely unusable. Then I have to restart it.

Eventually the length of time I can go between restarts shortens to just a few minutes. When this happens, I backup my home folder, delete the VM, and rebuild it. This restarts the cycle.
When this happens, CPU usage in the VM itself, CPU usage from my Mac, they're both normal. No spikes, nothing weird.

I've got a VM that's in rough shape right now. It can typically only be on for 10-30 minutes before a restart is needed.
Anyone have any ideas how I can find out what the problem is? Debugging tools, logging techniques, anything you've got I'm interested in.

In case it helps, here's what I do in the VMs:
* download / seed torrents for bundles I get from https://www.humblebundle.com/ (these are legal torrents, I'm not talking about piracy)
* use ProtonVPN
* browse the web using Firefox