I find Herbert Kegel's 1975 Parsifal to be, really, the ideal "fast" Parsifal, if one is going to play it fast. Historical precedent notwithstanding, I find a more moderate approach (i.e., about four hours to four and a quarter) to be more thematically suited to the drama. Parsifal by virtue of its dramatic content lends itself, in my mind, to a contemplative interpretation. Knappertsbusch and Kubelík had this in spades, Thielemann comes very close to that mark with the orchestra - only to be let down a little by his cast, and the rest can fall in line somewhere on the spectrum. Of course, it would be profoundly unidiomatic to do a Ron Popeil, pick a tempo, and leave the thing on autopilot. That is, of course, another discussion for another day. Kegel's Parsifal isn't quite as fast as Boulez', going strictly by the timings: there's a two-minute difference. That's trivial. So, I'll just say ceteris paribus, though Boulez has a better cast, the Bayreuth band, and the Festspielhaus acoustic going for his set, and say that Kegel has a more immediately interesting set. I don't think he handles clarity, texture, and rhythm as well as Boulez, but there is a visceral excitement to his set that isn't as apparent with Boulez.
That having been said, I prefer Boulez' set for the reasons I ignored with my equation above.