Hi Lars
So many reasons can be behind slowness, lets start with the Switch (the authenticator),there is a fast way to check, try to trace the server from client and see where its slow, then try without 802.1x from the PC.
If you configured RADIUS attributes to provide Vlan ID, check the switch's Vlan and port assignment.
aaa accounting may also cause a slow response, try to disable it and check the rersponse.
You can check also the integration between your RADIUS and the Active Directory, which can cause soem problems.
You can try to use the Local Switch's Database for 802.1x to see the response without RADIUS.
Good Luck