What does 'Fast mean?

Two points to answer:

"Fast is nonsense.... Same applies to PRAT."

No, Porziob, this statement is nonsense, at least to me. It is beyond me why people who have never experienced systems that do PRAT well insist that it doesn't exist. It most certainly does exist. Whether these systems are your preference is another matter entirely.

"Fast usually means the sound is thin and without body/weight making it seem faster! That is what I have found when auditioning "fast" gear."

Now I do not think this statement is nonsense, Grannyring. While I wouldn't put the matter in such pejorative terms, I think there is truth here. As far as I can tell the subjective experience elicited by systems that we say are fast or do PRAT well does seem to come at a cost of reducing the fullness of the sound, creating at least some degree of leanness in the music. What one gets in return is a musical tightness - a rhythmic coherence - that many of us find captivating and involving. Too much of this leanness can be as dissatisfying to some ears as bloated tube gear is to mine. (I don’t find all tube gear unpleasant - just the soft, mushy-mouthed stuff.) We all have our preferences.

Agree with Proziob.
Fast means nothing in electronics 'cause you should neglect the speed of a sound to a speed of an electron.

In speakers it's a proper mechanical response of a driver. The fastest speakers in this issue are heaphones period.

Stupendous amount of interesting (and some not so interesting) posts, few of which addressed the original question: simply, when one says "fast" what does one mean (speaking of reproduced sound).
Generally, it's what Seandtaylor wrote. Whether it's due to power supplies, correct hi-frequency reproduction, no ringing, no significant phase anomalies... is another matter.
Or, at least, Hayds forgot to ask.
BTW, "pratt" (lovely stuff from Linn's marketing) can be achieved in systems by slightly emphasising the lower mid-range/upper bass.

"BTW, "pratt" (lovely stuff from Linn's marketing) can be achieved in systems by slightly emphasising the lower mid-range/upper bass"

Gregm:
I own Linn gear, though I have never heard the term "PRAT" from the mouth of any of the three Linn dealers I have worked with in the past two years. But I am too young in the hobby to know the marketing history behind these terms. I actually assumed the term was Naim's coinage.

I don't understand the distinction you are drawing between the terms "PRAT" and "fast." At least I have been using them interchangeably, perhaps in ignorance. Could you explain what you mean? Certainly increasing midbass doesn't lead to the phenomenon Seantaylor is describing above.

In my experience, such as it is, the excitement, immediacy and rhythmic coherence of music clearly varies from system to system (most dramatically between some tube gear and good solid state). I am sure midbass can give you the “thump - thump” of a dance club, but I doubt that is what you mean. It is certainly not what I mean when I use the term “PRAT.”