Lol other people have done more pseudo-scientific testing, like here, and found that the rider felt like they were faster on clipless but were actually the same speed -

This video talks about a rider riding flats getting 2nd place in a pro race, and going through and trying it, and find that flats are faster than clipless:

The thing here is that he's doing downhill racing, where the advantage might be that there's less pedalling and more advantage to be gained by being able to put a foot down on the ground going around corners etc.

But I get the same office commuting to the office, for somewhat similar reasons - I don't have to waste time changing shoes when I get to work or going home from work. :-)

Racers get benefit out of clipless in situations like these -

They're sprinting for the finish line in an all out effort, bike is rocking side to side, the guy you're competing against is right next to you. That's the kind of situation where the additional foot retention of clipless is beneficial vs flats, where your foot is really being thrown around a lot on the pedal.