We are upgrading to GP 2010 SP1 from GP 9 and some users are suffering very slow performance.
After many hours head scratching it turns out the cause is due to .Net applications trying to authenticate via the internet. We have many users who are restricted from internet access by a group policy that sets their network settings to use a non-existent proxy server.
3-6 minutes were typical login times for non-internet enabled users, whereas internet enabled user login times were 10 seconds.
I found an article by Mark Russinovich that explained the problem and a work around by means of a .exe.config file with some xml that would cure the .Net Framework from trying to look at the internet to authenticate for non-internet enabled users.