Originally Posted by
surreycrv
It's the shoe, not the pedal, that makes for the efficiency. A carbon, or otherwise stiff, soled shoe will accomplish the goal. SPD's are walkable, so if you want to go into a store, pub, or even just a washroom without slipping, they're a better solution.
This is correct in my experience. I went from SPDs to SPD-SLs with the same brand / quality of shoe for each system, and could feel ZERO increase in efficiency. It's all about how stiff the shoe is. If you have a stiff shoe SPDs are fine. SPDs are so much easier to clip into as well.
I've been riding with SPD-SLs for a few weeks now and I've *mostly* gotten the hang of it, but plenty of times I still have trouble getting clipped in because the pedal is upside down and I have to make a second attempt while soft pedaling, which costs me time and speed. Because of this I'm pretty sure my average speed is a slight bit lower with the SPD-SLs.