In Total Guitar magazine some years ago it was suggested that the fastest playing was in the solo of Steve Vai's 'The Riddle' where he plays at a speed of 20.8 notes a second which is quite nippy.

My son went on an Extreme Guitar course in Bath recently and they pretty much concurred with the Practice, practice, practice thing. To develop speed play the exercises at a metronome setting making sure you are note perfect; develop physical memory of the piece; increase speed; and so on and on... Without precision speed being pointless. A friend of my son leaps into top speed mode first without doing the first bit and struggles.