Synchronous and Asynchronous Processing
Asynchronous refers to processes that do not depend on each other's outcome, and can therefore occur on different threads simultaneously. The opposite is synchronous. Synchronous processes wait for one to complete before the next begins. For those Group Policy settings for which both types of processes are available as options, you choose between the faster asynchronous or the safer, more predictable synchronous processing.
By default, the processing of Group Policy is synchronous. Computer policy is completed before the CTRL+ALT+DEL dialog box is presented, and user policy is completed before the shell is active and available for the user to interact with it.
You can change this default behavior by using a policy setting for each so that processing is asynchronous. This is not recommended unless there are compelling performance reasons. To provide the most reliable operation, leave the processing as synchronous.
Group Policy for computers is applied at computer startup. For users, Group Policy is applied when they log on.