In manny oracle 12c version the deferred_segment_creation feature does not work on table partitions, table subpartitions and lob partitions when you do a split partition. (ref. BUG 20307186)
This is very anoying if you have many empty segments.
There is an easy workaround.
Use DBMS_SPACE_ADMIN.DROP_EMPTY_SEGMENTS to drop the emptry segments again after the split partition action.
Actually this does not give any speed benefits while splitting partitions, but it may save you from space problems with emptry segments.