smartctl recently helped me nail down some nasty slowdown issues with Proxmox running on an Intel NUC. the problem turned out to be that the box was running really hot!
smartctl -A /dev/<device> | grep 'Temperature:'
Do you have a habit of checking your disk health? I check mine every month to prevent disk corruption and data loss.
If you are interested in smartctl, here is a demo: https://x-cmd.com/1min/smartctl