See the Explain text for that policy setting for details. Group Policy affects only users and computers contained in sites, domains, and organizational units. Specifically, Group Policy objects are not applied to Security Groups. 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. Note 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.