Have you enabled swap ?
It's quiet the opposite in my case, after disabled swap I haven't met this problem.
CPU(s) 12 x AMD Ryzen 5 3600 6-Core Processor (1 Socket)
Kernel Version Linux 5.3.18-2-pve #1 SMP PVE 5.3.18-2 (Sat, 15 Feb 2020 15:11:52 +0100)
PVE Manager Version pve-manager/6.1-7/13e58d5e
Code:CPU(s) 12 x AMD Ryzen 5 3600 6-Core Processor (1 Socket) Kernel Version Linux 5.3.18-2-pve #1 SMP PVE 5.3.18-2 (Sat, 15 Feb 2020 15:11:52 +0100) PVE Manager Version pve-manager/6.1-7/13e58d5e
Try to update zfs to 0.8.3, there were a regression that disabled hardware checksum calculation.
Did you succeed with updating Proxmox / ZFS?After a restart the server is working fine, next window I will upgrade the Proxmox / ZFS to last version.