What makes a sytem "Fast?"

I believe it is the transient response of the Amp that makes it fast. Slew Rate

I don't understand the "how" of fast but will share my experience, for what it's worth. When I inserted a Supratek Syrah tube preamp into my solid state active system I was taken into another dimension. Very difficult to describe the way my amps were unleashed to do what they were meant to do. You could have knocked me over with a feather the change was so pronounced. I'll be happy to read to technical aspects of fast when the pros chime in.

"Fast" or "speed" should refer to rise times and settling times of impulses (as it regards individual components and the system as a whole, including the room). Personally, I suspect that for transient attacks, it is really more a lack of dynamic compression than rise-times per se that gives an impression of speed. IMO it is actually the decay aspect that most distinguishes "fast" from "slow" sound, where overhang or resonances that obscure clean impulse decay leads to an impression of "slowness" (maybe in conjunction with compressed attacks and/or an overly reverberent or strongly modal listening room).