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:

<b>             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.</b>
On a BDC, Event Id 5715 is logged:
<b>             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.</b>
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.