Clipless vs. Flat Pedal Speed Difference
With flat pedals, the only thing holding your foot to the pedal is friction. Friction only exists if your foot is in contact with the pedal -- what physicists call &quot;normal force&quot; or &quot;contact force&quot;. No force, no friction, and your foot is off the pedal, by definition.
Because of this fact, you have to exert some amount of pressure to the pedals all the time, particularly on the back-stroke...unless you have super Jedi warrior reflexes and can move your foot in exactly the same arc as the crank, your foot is either going to come off the pedal, or it's going to have to be pressed against the pedal at least a little bit to maintain traction. This &quot;little bit of retarding force&quot; directly counter-acts the main driving force of the other crank...you are literally working against yourself; your legs are working against each other...and this extra effort that must be expended to maintain traction also won't show up in a power meter, depending on the design of the power meter.
It's a physical fact that flat pedals require you to keep your foot pressed against the pedal, and this backward force generally is non-productive and actually detracts from forward force. The main argument is how significant of an effect this is. There have been studies to measure the difference, but not all of them directly measure the right things. The current consensus is that the effect is small, and efficiency for most practical purposes is not greatly different between flat pedals and clipless/clipped pedals. To directly answer your question, I believe attempts to find a difference through practical testing (the straight road, direct comparison test) have failed to find a reproducable difference.
It may be a difference too small to matter, but other things considered, clipless should be more efficient because the need to apply this retarding force to the cranks is removed.