So a couple of things I'd be looking at if it were me:
- RAID card config: write-through / write-back will have performance impacts (but should hit S2019 and W10 equally)
- Network vs storage:
-- iperf3 only runs in memory, so it completely removes storage from the troubleshooting equation, if you see the same type of drop-off testing with iperf3 you know that there's a networking gremlin somewhere that needs to be dealt with.
-- something like LANSpeedTest actually writes and reads a file on the far-end storage, so it should provide the same results as your typical file transfer, you can also arbitrarily set the transfer size, just in case you want to test something bigger than what you've got as a static file.
- What's actually running in the OS at the same time
-- use something like processhacker to see what else might be using the network or other IO when your file transfer slows to a crawl.
--Maybe there's security configs being applied to your servers and not the W10 guests that aren't being taken into consideration.
I'm not sure if these will help...
The physical servers over the network work fine. Full speed ahead! So, cant be RAID settings, network issue or storage. Physical <-> Physical is perfect. What is the point of testing with iperf, im saying already physical <-> physical is perfect...
The issue is with the VMs. From a VM on host A to a VM on Host B, im seeing much slower speeds. From physical A to physical B, its fine.