Check what all happens when the user logs in. Things that come to mind:
- User's profile is downloaded each time to the local machine and it is huge
- Network scripts are run and they are either erroring or slow
- User has things in their roaming Startup group that takes forever to run
Since it is across machines then it most likely isn't a local system issue but check the event viewer logs to see if it has any errors from the user logging in.