The defautl vm.swappiness value for Proxmox 5.2.x is 60
Run this command in Shell in order to reduce swappiness to an absolute necessity when the system is out of RAM.
sysctl -w vm.swappiness=0
https://forum.proxmox.com/threads/disable-swap-on-host.25623/
https://forum.proxmox.com/threads/proxmox-is-using-swap-with-lot-of-ram-available.41994/
Other performance tips