A slow logon process can come from various cause :
The user may take some time to authenticate on the AD.
When a logon script is executing and mapping network ressource, it can take time to perform the task.
This can be checked in msconfig. You can also open a task manager to monitor precisely which processes are run by each users. Doing so you should be able to narrow down the ones that are responsible for this.
At each logon the server may look to make a peripheral available for a user, slowing down the logon process.
You can choose to un-check drives and peripherals that are not needed in the local ressource tab of the client generator.