One thing I've noticed whilst learning to play an instrument is that, and it came as quite a shock, was that the professionals I enjoyed listening to didn't play anywhere near as fast in real life as they did in my head. Therefore I was trying to play like the pros but was actually racing ahead of them. Perhaps this is not unique to me and contributes to the rise in the speed?

I can't comment on all forms of dance, but the one's I do or play for (English Ceilidh and morris) is that it can be as bad playing too slow (too much effort to step properly) as to play too fast (rushed and scrappy). Any dance where you need to leave the floor unassisted (unlike the volta) must not be too slow!!