@olivierlambert Thank you!

Do you actually "feel" the difference in terms of VM performance for your services?

OF COURSE! Was very slow.
You cannot imagine the difference...

This is an AMD Ryzen 9 7950X3D host.
Moreover, since the problem started we migrated several VMs to other hosts.
If you see the screenshots above, in xentop output, besides Dom0, all other VMs where like hardly using 2-3 CPUs in total.
And everything was dead slow (steal time always high).

My feeling here is that this was somehow related to networking (tap*), but really don't know what and why.
Glad this was resolved after the reboot.

Sometimes, I had to try 3 or 4 different sticks before one works. Those little things tend to die pretty quickly.

@tuxen Thank you! I will try it