There are plenty of players who can play fast on the harmonica, and in the forty-plus years I’ve been playing the general ability of harp players to execute fast lines has gone way up. Players like Sugar Blue, John Popper, and Howard Levy showed pretty conclusively in the 1980s and 1990s that previously accepted speed limits were no longer applicable, and almost everybody plays faster now than they used to.
In general, it’s difficult to play harmonica as fast as, say, saxophone, because it’s more difficult to rapidly change direction of breathing than it is to move your fingers fast. So I doubt that any harmonica player will ever set the world’s record for speed for all instruments.
Some of the posts to this thread basically say “why bother?” Which begs the real question: why do we want to play fast? The short answer is that there are certain emotional effects that you can’t get any other way–not just the awe that’s produced in an audience when you play fast, but more subtle things as well. Notes played fast and hard, the way Popper does, hit like a hammer blow; notes played fast and less aggressively are an intricate web of ideas surrounding and entrancing the listener.
So it’s great to be able to play fast. However, on ALL instruments, there is a tradeoff: the faster you play, the less the audience hears the emotion on every note. You can blow their minds with speed, and there’s nothing wrong with that; but if you want to break their hearts you’ve got to slow down.
Howlin’ Wolf could blow your mind AND break your heart with a single note, of course, as he does in the video below. But there’s more to music than any one of us can encompass, and there’s a place for fast playing in that universe.