Whenever I'm learning a new tune, I play it slow, fast, and in between. Also on a couple or three different instruments. Most tunes teach me something new about themselves at each speed and on each instrument I try it with. Sometimes the feel that comes out when I play it fast can be transferred to the slow version. Sometimes I end up with two completely different arrangements of the same tune and can plug them in according to my mood.

As soon as there are no dancers there is no particular reason to play a tune at any particular speed. The point is to make it sound good. Most traditional tunes sound better to me at a slightly more relaxed tempo than the speed demons want. The thing I like about traditional tunes is the melodies. If the melody is going by too fast to appreciate the nuances, what's the point?

Anyone who speeds up a tune that someone else started is just displaying their inexperience and lack of musical cooperation. Anyone who plays faster than they are competent to play should just stop.

Really good players can play a tune extremely fast with all appropriate ornaments in and without losing the throb. Most of us can't, and shouldn't try. As soon as you lose the throb you may as well put your instrument down. I use that as my limiter for the upper end of the speed on all tunes. That doesn't mean that I always play them as fast as I can; it just means I won't play them faster than that.

When I play for dances, I play at appropriate dance speeds and adjust if needed by watching the dancers. Knowing how to dance the dance can allow you to add subtleties to your playing that only mean anything if you know the dance, but which add a ton to the experience for everyone involved. I'm thankful that I've rarely been called on to play for Irish dancing, since the tunes are mostly played too fast for my taste. They lose their musicality. I've played a lot for French and Swedish dancing. Much more sensible speeds.

John