Don't think it's anything to do with fast/slow, just basic manners/personality.
I've seen rude slow runners too and I've seen polite, encouraging and great fast runners. I did a race a while ago and the winner waited at the finish until pretty much everyone had come in, congratulating each runner as he/she passed the line.
OK, it was a local race, but even so
.
I'm nowhere near a fast runner, but if I was pushing myself as B says, for whatever reason, I wouldn't want to stop (nor would I) for someone congratulating me, those few seconds can be make or break.