Rohit is right in the direction meaning that if PCTUSED and PCTFREE are not set properly, you can waste time placing blocks onto and taking blocks off the freelist.

Run

Code:
select table_name, NUM_FREELIST_BLOCKS from user_tables;
to see the number of blocks being placed on and taken off the freelists.

This is my opinion: if you NUM_FREELIST_BLOCKS is more than 1% of the blocks of the table, then you may have a problem. May have!