GCN did a couple of clipless vs flat pedal comparisons and found no significant (for mere mortals) measurable or observable differences in power or speed.
I haven't noticed any difference in my speed over the past couple of years since switching to clipless for my road bikes.
Main difference I notice is clipless feels more efficient over longer distances and elapsed time rides. Fewer problems with muscle cramps in my feet and thighs. Less tiring. Spinning with clipless seems to be more efficient, but not faster.
Pretty much the same thing I noticed back in the 1970s comparing toe clips and straps with cleated shoes, vs platforms. The only difference was on longer rides.