SMBv1 is disabled by default and it could be causing this kind of issue. Being this is a legacy program, that would be where I would look at. Enabling SMBv1 on this 2019 VM and see if the performance issue is addressed.
SMBv2 is supposed to be a bit faster than SMBv1.
Yeah, it is, but the clients have to communicate to the server, realize the server no longer supports SMB1, and then connect via SMB2 or SMB3 if the server supports it.
This is also affected by the "Access" style client/server which may only operate with SMB1 in mind.
With this logic, you should then be disabling SMBv1 on the clients. That is even if your clients still have SMBv1 enabled. They would mean someone hasn't been paying much attention to vulnerabilities over the past 2 years.
Scott migrated a legacy application from 2012 to 2019. That's 7 years of change. Assuming the customer didn't actually update the client software, which was likely developed with SMB1 only in mind.
But I get the argument, in this case I think the client has caused more problems than the protocols used.
I just dont think turning SMBv1 is even a consideration anymore in 2019.