When I first got clipless, I felt like they were way faster. But recently I started experimenting with switching back to flats, and found that I would often feel faster on clipless - but not actually be any faster looking at my speedometer. So I'm not sure if there's any real speed advantage or not. I've been riding with clipless for years, and went through adjusting how I pedalling with a fitter and everything.

What clipless seems to achieve well is foot retention. You're not spinning at high speeds, or riding in the rain, and have your foot coming off the pedal.

They used to use straps and racing where you would physically attach your foot to the pedal with straps that you would reach down and tighten. "clipless" replaced this, and clipless is far better than a toe cage at letting your foot disconnect from the pedal in an emergency. It's not flawless, but it's far far better at it vs any sort of toe clips.

I do think "pedalling efficiency" with clipless is both overhyped, and clipless can give you a feeling that you're getting a lot more power when you're not. Whether it gives you a little more power is debateable, and I don't think there's a reasonable conclusion one can reach on that topic on the internet. They are very good at keeping your feet attached to the pedals in a way that mostly lets your feet unclip in an emergency, though.