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>>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.

With SMB2, the first user gets exclusive use of the files. So naturally it's fast. When the second user accesses, all buffered data is flushed, and the access switches to shared. That cache dump and switch can be really slow. It's complicated by opening and closing files, because if only one user's machine opens a table, they have exclusive access. Depending on the usage of the app at one company, there may be many cases of exclusive access, while at a second company there may be many cases of shared access and even switching.

That's why I've opted for consistency. SMB2 is off. All tables are opened at app start. There is never any cache dump and switch, and there is never any exclusive use. Then I only have to focus on every other optimization I can squeeze from the code. :)

There is no way to get shared access to be as fast as exclusive as there is no way to get accessing a table on the server to be faster than accessing the table on the local drive.