Windows Server 2019 Hyper-V guest disk performance

I have a site who run a very IO-sensitive application (Accredo Saturn); it's an accounting/CRM package written in Delphi with a local flat-file database.

For various historical reasons the site was running it on a Windows Server 2008 R2 terminal server running under Server 2012 R2 Hyper-V on a Proliant DL380 G9, with their DC being an old DL380 G7 with SBS 2011 (Exchange has long been on Office 365).

I've now upgraded them to a new DL380 G10 running Server 2019. The host and domain controller are running on 6x 600 GB 10k SAS in RAID10 (host on its own partition, one big partition for the rest) on a P408i-p, with the remote desktop server on 4x 480 GB mixed use SATA SSD in RAID10 on a P408i-a. The server has 2x Xeon 4210 and 64 GB memory. The data for this software is on a VHDX on the SSD array, mounted directly on the remote desktop server.

They have 18 users, all of whom use the remote desktop server for this program, with 8 callcentre users also using a Unify phone system agent. One or two use Edge. I intended to go a bit overboard on specs because this client is fussy about speed, and as I mentioned the software is fussy!

The client has complained about slow speeds within the software. I've tested, and found that an operation that was taking 5 seconds is now taking up to 15. The old 2008 R2 VM on the same hardware is performing as it always did, so it almost seems to be something to do with the guest.

I have run diskspd with no users logged in (-c100b -b4K -o32 -F8 -T1b -s8b -W60 -d60 -Sh) and am seeing similar read IOPS and throughput on both VMs, but with some wide variations in threads on the new 2019 VM. I'm seeing about 531.41 Mbps and 136k IOPS on the guests, but two threads down at 1.9 Mbps on the 2019 VM. The old VM is sitting at 520.44 Mbps but consistently around 72-76 per thread, except for one thread down at 3.75. Total was 133k IOPS. That's on the SSD array.

In comparison the bare metal SSD array with the same parameters is giving me 999 Mbps, consistently 124-125 Mbps per thread and a total of 255k IOPS.

I have been days looking into this. I've tried the registry entry to disable the IO load balancer, but to no effect – I'm not sure if it even applies to 2019. I've tried both fixed and dynamic VHDXs – even swapped the data volume between servers (it is its own VHDX). Have tried dynamic and static memory. Have tried NUMA enabled and disabled.

I'm at my wits end and have a frustrated client who is starting their callcentre for the year on the old VM tomorrow!

The 2008 R2 is a generation 1, version 5 VM while the 2019 is generation 2, version 9.

Any hints on getting those mission IOPS back would be appreciated!

This is my first post, so apologies if I haven't included enough relevant or specific information.