I'm talking about transient response, the actual speed of the drivers moving through the air, back and forth, creating sound. Not how bright they sound. I get the feeling when you think "fast" you're thinking more potent treble, like Grado, if not my apologies. With giant neodymium magnets on both sides of a diaphram creating a huge mangetic flux I think they move pretty damn fast, faster than everything except other planars and stats. Maybe the ring radiator from Sennheiser is faster too, but that's about it. So yeah I would consider them pretty damn fast. Faster than tesla and faster than even balanced armatures in IEMs. At least that's how I hear it.
I was talking about how energetic/speedy different headphones sound. With my HE-400 I feel more energy and speed. I don't know the technical reasons behind it.