Speedplays and SPDs might be cousins, but they’re definitely not the same. Unlike SPDs, which have a small solid cleat on the shoe and a spring-based retention system on the pedal, Speedplays have a large surfaced cleat on the shoe that includes a wire spring while the pedal has no moving parts at all. Speedplays are remarkably light compared to Look and SPDs, and because of the relatively large surface of the cleat, I’ve not heard of anyone complain about hotspots on their feet like they might with SPDs. But really, the very best feature of all in my opinion is the smooth, very adjustable float that has always kept my knees quite happy. You can also purchase longer spindles if your fit requires a wider pedal stance than you could get through other means.
Here’s a link to Speedplays at Competitive Cyclist: https://www.competitivecyclist.com/speedplay-zero-chrome-moly-pedals-with-walkable-cleats
and for comparison sake, some SPDs: https://www.competitivecyclist.com/shimano-xt-pd-m8000-xc-pedal?skidn=SHI008Y-SPD-ONESIZ&ti=U2VhcmNoIFJlc3VsdHM6c3BkOjE6MzpzcGQ=