I recently recommended a complete bicycle overhaul to a customer before his first Ironman, and he opted not to postpone it until after his race (against my advice).  Post race he brought his bike in for the overhaul which included new cables, crank/cassette/chain, and service and/or replacement of bearings.  His bike was in fairly deplorable condition with excessive wear throughout, and had not been cleaned or de-greased in ages.  Once I removed the chain I was surprised to find the crank was fairly immobile, like that of a spin bike under tension.  He had basically raced his entire race with his power being "sucked up" by his drive train.  There is no telling how much time this added to his bike leg- 10, 20 minutes, maybe an hour...

Drive train "suck" comes from various places but the main culprits are the chain, pulley wheels, and bottom bracket.  Athletes will spend hundreds on ceramic bearings to reduce a small amount of resistance from these areas and gain a few watts.  However, a dirty drive train or worn bottom bracket produces far more drag.  A clean drive train helps a lot.  Grime accumulates in certain areas, especially around the pulley wheels, and you can literally feel the difference between a clean and dirty drive train.  But you cannot see a worn bottom bracket and it is important to get this inspected, serviced, and/or replaced at least once per season.  You can check for excessive wear by hand pedaling backwards.  You may perform this test with the chain removed from the inside cog and it will help isolate the bottom bracket resistance.  If there is any grinding, play, resistance you should have your bottom bracket inspected.

Athletes spend hundreds, even thousands on ways to optimize their Ironman performance.  But overlooking a $40 bottom bracket may cost you a lot more in time off your bike split.