If you change frequently, adding and/or deleting new members to a very large domain global group, replication can take a very long time.
You may observe a large number of Event Id 5711 for BDCs on your PDC:
Description: The partial synchronization request from the server BDC completed successfully.
nn changes(s) has (have) been returned to the caller.</b>
The partial synchronization replication of the SAM database from the
primary domain controller completed successfully.
nn change(s) is (are) applied to the database.</b>
If you must make these changes, do them all on the PDC. When complete, force each BDC to fully replicate using the NLTEST /SYNC command on each BDC.