If the table being altered is partitioned, parallel sorting can be used while rebuilding the indexes. alter table performance can be greatly improved if the data cache and server are configured for optimal parallel sort performance.
During alter table, the indexes are re-created one at a time. If your system has enough engines, data cache, and I/O throughput to handle simultaneous create index operations, you can reduce the overall time required to change locking schemes by:
Droping the nonclustered indexes
Altering the locking scheme
Configuring for best parallel sort performance
Re-creating two or more nonclustered indexes at once