As you already know, Group Policy processes settings during computer startup and shutdown, and user logon/logoff. It also does a background refresh cycle every 90 minutes as the users are working on their computers throughout the day. The goal for Group Policy processing is to be as fast as possible, hopefully measured in milliseconds, so that your employees are not wasting time sitting around and waiting for these settings to finish processing.
To this end, in a normal GPO background refresh cycle, the computer evaluates whether or not GPOs have been modified. If they have since the last cycle, then of course the computer needs to spend time and energy processing through those modified GPOs. But ...
