Fast should mean quick transient response, but nothing can be taken out of context: you can't, as audiophiles do, find a fave word and run with it. Fast could mean an amp with a high slew rate, then someone will point out that some amps with a low slew rate sound terrific. Do you judge the beauty of a women based solely on, let's say, the nose? A system is a chain of many components and each component is the result of many choices or compromises. To say that fast means transparent gives pretty short shrift to distortion and noise.
