A client of mine is using Quickbooks 2009 and has been having issues with accessing Quickbook files hosted on the Quickbooks database server. All firewall ports are open, the database server is running and the Quickbooks Connection Diagnostic Tool says everything passes. They access the files from a mapped drive to the server. It's never ran or performed as well as I think it should on a gigabit network with fairly new machines.
After doing some research I tried accessing the Quickbooks files over a UNC path rather then a mapped drive. The performance difference was night and day. The files opened nearly instantly and it ran as if the QBW files were locally stored.
From Intuit's website...
Note: In prior versions of QuickBooks, Intuit recommended that a mapped network drive be used to access QuickBooks company files. With QuickBooks 2006, the UNC path, or direct path to the company file will work better than a mapped drive.