Yeah, it did slow me down. Might have cost me the race.

And any study that say you (well, me anyway) don't produce power on the upstroke is seriously flawed (see "pulling out during a sprint").

That said you can still go pretty good on platform pedals sitting down. Standing to climb or sprint is a different story. If you're a casual rec rider clipless might help, or it might be very bad depending on how you're set up.