Quickbooks on server

Is this QBs Enterprise or are you just QBs Desktop in a Multi-User mode?

QBs should really not be putting that much demand on your Network or Server, but then I don't know your setup.  The first checks should be the Server and Network Logs to see where the bottle neck is.  Also a quick wellness check of the Server, its Drive, etc. 

If that does not show anything definitive and you're just using QBs Desktop, another relatively quick test would be to put the QBs Company File on another Computer in a Network Shared Folder.  Then just re-point the other users to that folder and see if the performance improves.