We deployed a new Azure Virtual Desktop host pool that only contained Entra ID joined session host VMs, same NVIDIA GPUs. upgraded to Pro 3.2. Since the VMs were no longer AD joined, we had to migrate all the AD Group Policy Objects to InTune, which really wasn't that difficult given the GPO import tools in InTune.
FsLogix profile containers were switched away from Azure Files Premium and are being stored in an Azure NetApp Volume, just like our GIS file-based data.
Entra ID-joined session hosts can only authenticate with SMB shares using Kerberos authentication, not NTLM (on-prem AD). So, Kerberos auth. and ACLs (permissions) were configured per Azure docs. for both the NetApp \profile and \gis volumes.
For the results: ALL PROBLEMS ARE SOLVED!
Added bonus is that all MS office apps, including the Remote Desktop client are SSO using Entra ID (cloud) credentials. The VMs, Pro, and FGDB access is 100% improved.
I wish I had a better explanation, but our problems with FGDB performance in AVD was 100% caused by authentication issues with traditional on-prem AD (which was a VM in Azure, not really on-prem).
With the sputtering/performance issues resolved, I have no doubt that better performance could be achieved by increasing the NetApp capacity pool quota (i.e. monthly A$ure invoice). Our current cap. pool quota is 2TB which comes with 128 MiB/s throughput, divided how ever we want between \profile and \gis volumes. Another 1TB would add +64 MiB/s throughput.
Azure NetApp is quite amazing, so simple.