I just stumbled across the following post: http://techibee.com/active-directory/slow-login-issues-due-to-group-policy-preferences/1096
Looks like this guy suggested a way to speed up the evaluation of user/computer groups with group policy preferences (therefore speeding up logins and boot ups) at the AskDS site and they implemented it (http://blogs.technet.com/b/askds/). It seems this will only speed things up if your using security groups in your item level targeting.
This is what they produced: http://support.microsoft.com/kb/2561285
I’m certainly going to test it and see if it’s worth rolling out where I work.
There are a few other tips on speeding up group policy processing generally that can be found here: http://www.frickelsoft.net/blog/?p=66. In summary disable portions of a policy you don’t need to use (user/computer configuration), try to use the least amount of policies you can, and don’t overuse WMI filters.