On a hunch, can anybody try with the 5.15.35 kernel while turning some spectre/meltdown security mitigations off (shouldn't be done if you host for untrusted and/or third parties).
You'd need to add
mitigations=off
to your kernel boot command line
https://pve.proxmox.com/pve-docs/chapter-sysadmin.html#sysboot_edit_kernel_cmdline
Then don't forget to
update-grub
or
proxmox-boot-tool refresh
and also do a full host reboot.
Because I could reproduce a Windows issue on an older host with outdated FW that gets fixed by that (not the whole spectre/meltdown patches are at fault, just some extra ones that got in with 5.15.27-1-pve that I'm still investigating more closely), while the symptoms are are partially the same as described in this (and other similar) thread, it's a bit hard to tell for sure that this is the exact same problem most of you are facing, which also may be more than one issue.
Note that in our case we got an identical host with a newer, up-to-date BIOS/Firmware, which also fixes the this particular issue on this particular HW.