I deviated onto flats a bit recently and SPDs are faster and more comfortable for me. While I’ve got shoes that grip like mad, I can never seem to accurately replicate the position I get on SPDs, and when I put my foot down there’s a lot of shuffling around finding somewhere that’s comfy. Obviously SPDs eliminate that.
SPDs are faster for me too- I find I have more control over the bike, but I can put more power down when I need to. By way of evidence, I did a DH race about 3 years ago with flats for one run and SPDs for the 2nd and the SPD run (the only variable was the pedals) was around 8-10 seconds faster.
I do use flats still for rides when there’s going to be a lot of walking but for me, SPDs trump flats hands down and it’s purely a fashion thing that people are changing them for. While some people will genuinely benefit, your average overweight middle age punter won’t. Even for DH.