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:

             Event Id 5711 - Source: NETLOGON                                           Description: The partial synchronization request from the server BDC completed successfully.                                           nn changes(s) has (have) been returned to the caller.
On a BDC, Event Id 5715 is logged:
             Event Id 5715 - Source: NETLOGON                                           The partial synchronization replication of the SAM database from the                                            primary domain controller completed successfully.                                           nn change(s) is (are) applied to the database.
The PDC CPU usage may remain at 100% during replication. The amount of data being sent may be greater than the size of the SAM. You can monitor this using the NBTSTAT -S command.

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.