Microsoft Knowledge Base Article 315418 contains the following summary:
This article describes how to optimize and configure Group
Policy to increase logon performance.
When you start a Windows 2000-based computer that is a member of a domain, group policy settings from the "Computer Settings" section of linked Group Policy objects (GPOs) are processed and applied to the computer. Additionally, when you log on to the domain, all group policy settings from the "User Configuration" sections of each linked GPO are processed and applied. Because Windows takes time to apply each policy setting, policy settings may slow the logon process, which can result in a delay from the time that you start the computer to the time that you are able to use the computer. This article describes methods that you can use to minimize this delay.