It sounds like your app is running on workstations. Are your database files heavily fragmented? Have you tried to shut down your app and SQLExpress (just stop the service) then thoroughly defrag the drive?
If you have two (or more) physically distinct drives in the systems, have you tried to move the database logs (*.ldf) on a different drive than the data files (*.mdf)? Putting the transaction logs and data files on the same physical drive is a huge performance