>>
>>Have you made any progress with your performance issues?
>*
>Since we could not figure out the cause of the problem, I decided to address the symptoms and make sure that all SQL queries /JOINs are optimized, which helped. The problem seems to be related to file locking (as soon as the 2nd user is on using the application, the performance slows down 10 fold, and it remains as slow that regardless of the number of additional users). It is an ongoing issue right now; I may come back here with more news. Anyway, what's puzzling is that we see this only at a couple of clients; everyone else is fine.
>Thank you all for your suggestions.

As Mike mentiones, this is typical for SMB2 - but also for enabled oplocking, which can happen without SMB2. Too lazy to check if that was mentioned in Franks settings.