If pings to a VM from a device on the same local network, over ethernet are inconsistently higher than <1ms, for example:
Check if the HyperV host has Broadcom ethernet adapters and Virtual Machine Queue (VMQ) enabled (this is default on broadcom).
Disabling this feature should resolve the issue:
(in this case the VM was pinging it's HyperV host while the change was made)
It's possible to get the benefits of VMQ and eliminate the issue by using a reg key such as here:
After 2 years, I have finally solved my "Slow Hyper-V Guest Network Performance" issue. I am ecstatic. : r/sysadmin (reddit.com)
But in general people seem to just disable VMQ on broadcom NICs as standard for HyperV installations.