I've ridden platforms and clipless on my old school steel road bike, and my early '90s Trek OCLV for a week or two before settling on SPL-SL rather than Look Delta pedals.
Checking my ride data, no significant differences in speed. Some of my fastest Strava segments were set with platform pedals on those bikes, while I was wearing Merrell shoes (not nearly as stiff soled as Five Ten). Those include flat terrain, rollers, climbs and downhill segments.
But I tend to have a choppy cadence and mash more than spin. That might matter.
Back in the 1970s, when men were etc and blahblahblah, I used Detto Pietros and toe clips, strapped in and snugged down. Frankly, it was awful. The cleats were way too far forward for my feet, on the balls of the feet per the Conventional Wisdom of that era. I had arch spasms, shin splints, etc. I swore off foot retention after several years of that mess. But it was mostly a bike fit problem (nobody seemed to know anything about bike fit back then, other than what they memorized from Saint Eddy) and my physiology, not a knock against foot retention in general.
When I resumed cycling in 2015 I got oversized platform pedals and whatever walking shoes I already had and was perfectly happy for a couple of years.
Until I got my first road bike in 30+ years in 2017. Then the old competitive streak flared up again.
Main reason I eventually switched to clipless on the road bikes was for better support for my feet. My feet are terrible, bony with high arches. Since I was a kid I've had problems with painful cramps in my arches and legs from running or cycling. Boot camp was miserable with those cheap gym shoes -- I ran better with boots, a trick I learned from watching Muhammad Ali train. He jogged in high top military boots, including running backward. Worked for me as well, just because of the stiffer soles and better arch support.
I might be very slightly more consistent over distance with clipless. Occasionally when I feel spasms coming on in my arches, calves or quads, I'll switch to a lighter gear, spin at 90-100 rpm (I'm usually around 60-75 rpm), consciously pulling up on the pedals until the spasms fade. Worked great the other day when I was chasing some hill climb PRs. Seemed to conserve a little energy. But it's only maybe a 1/2 mph difference in average speed over 20-40 miles. I mean, I'll take it. But it's not a huge difference.
Last year I got a pair of iSSi Thump pedals for my favorite hybrid. Love 'em. I'm seriously considering switching my old school steel road bike to similarly wide but lighter weight flat pedals and retiring the old Look Deltas. Occasionally I ride that road bike toward the city, and might use the MUP for a mile or so as a shortcut. I dislike foot retention in traffic (which includes the MUP, since I always defer to the pedestrians). It's just easier to dab a foot when I don't need to unclip or yank free of toe clips and cages.
And if we resume casual group rides after the 'rona virus pandemic settles, I prefer flat pedals since we do some walking, or stops at pubs or for coffee. I like Look Delta, Keo and Shimano SPD-SL for riding, but they suck for walking more than a few feet. It's like walking in high heels designed by Salvador Dali. Nobody takes me seriously when I clippityclipclop on my toes up to the bar for a manly man's ginger ale or kombucha. I'm sure the Lycra and Spandex have nothing to do with it. It's the shoes.
But I'll stick with clipless for my other road bikes. As I said, I don't see any data showing I'm significantly faster with clipless. But it does help minimize my problems with arch and leg cramps. That's good enough.