Hi Chris,
Most of the 20 clients are still running fine, but a handful (3 identified) at the moment are running really slow, they are only running slow in the VFP application any other use of the server like copying files runs fine. They also seem to be the better spec'd machines out of the 20. Nothing has changed on the client pc's other than the unc path where the application is looking for the data.
I had issues with systems were the number of TCP/IP packets that could be exchanged between client and server depended on the version of Windows on the server. So the same client would exchange 250 packets per second with a Windows 2012 R2 but 1250 with Windows 2008 R2. Because VFP requests records one at a time and only reads the next one when the first has been processed, we ended up reading a maximum of one record every 4ms or every 0.8 ms. Both were slow, but the new Windows server considerably slower.
My first step would be to run Process Monitor on the client once for the faster machines and then once for the slower system. Then look at the timing between ReadFile requests in some table. Preferably one where multiple records are read in a row, such as a SCAN loop, SELECT statement or a grid control. Look if there's a significant difference in the duration between two records and if this number is somewhat constant.
If this doesn't help, then WireShark is a great tool, but does require some digging into SMB protocols.