You may experience slow network speed with Hyper-V Virtual Machines on Windows Server 2019. This issue has been caused by Microsoft’s new feature called “Receive Segment Coalescing (RSC)”. However RSC in the vSwitch is a feature in Windows Server 2019 and Windows 10 October 2018 Update that helps reduce host CPU utilization and increases throughput for virtual workloads by coalescing multiple TCP segments into fewer, but larger segments. Processing fewer, large segments (coalesced) is more efficient than processing numerous, small segments.
You can get more details from Microsoft.
According to Microsoft, by default on Hyper-V external switches RSC is enabled. You can view this by below command.
Get-VMSwitch -Name <vSwitchName> | Select-Object *RSC*
Disable RSC and check for the network issue.
Note: RSC in the vSwitch can be enabled and disabled on the fly without impact to existing connections.
Set-VMSwitch -Name <vSwitchName> -EnableSoftwareRsc $false
If you want to re-enable RSC you can use the below command.
Set-VMSwitch -Name <vSwitchName> -EnableSoftwareRsc $True