Thanks for all the discussion! Especially the very detailed response from eoinnz. I did also see the CNN article the day or so after I posted my question. I also followed a link to a YouTube video of a practice evacuation of, I think, a 777. It was something with a 2-by-2 slide.

I would think in an emergency people would be pretty willing to jump and go, especially if everyone else was, assuming the area outside was (relatively) clear compared to inside. But jumping onto a slide isn't an instinctive way to exit which is probably the issue. The one time I went tandem sky-diving, I found that when I was at the edge of the jump door, jumping just sort of seemed natural, so I can imagine that might be the case in an emergency. But then again, maybe not. Hopefully I'll never have to find out!