One thing that can speed up Group Policy processing and logins, is to disable the User or Computer portion of a GPO if there are no settings being applied.

As an example, if you have a GPO for your an OU that contains only computer accounts, you can disable the User portion of the policy as its setting would not be applied. The same is true for an OU with only user accounts. To disable the User or Computer portion of a policy, right-click the OU and select Properties. Click on the policy and press the Properties button. Tick the box for the portion of the policy you wish to disable.

You will be given a warning, click Yes to accept the change.