Very poor network performance with Server 2019

You may be seeing this due to Receive Segment Coalescing (RSC). In short, RSC is TCP segment aggregation that combines segments in order to process larger segments versus multiple small segments for efficiency. This was introduced as a default configuration change for Windows Server 2019 and has been the source of issues similar to yours in other environments.

1. You can disable a client's RSC for IPv4 traffic using PowerShell: Disable-NetAdapterRsc -Name $nic -IPv4.

2. You can check a Hyper-V vSwitch's current RSC status using PowerShell: Get-VMSwitch -Name $vSwitch | Select *RSC*.

3. You can disable a Hyper-V vSwitch's RSC in full using PowerShell: Set-VMSwitch -Name $vSwitch -EnableSoftwareRsc:$FALSE. This will not impact existing vSwitch connections.

Items #2 and #3 require you to have access to the Hyper-V PowerShell modules and proper Hyper-V management permissions. As you mentioned seeing this across multiple machines on a Hyper-V vSwitch it is likely that item #3 is where you may find relief. I've provided reference material for you here and here.