What does 'Fast mean?

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.

They're hyperfast
--guitar
Al DiMeola, John McLaughlin, Paco DeLucia...
Ingwie Malmsteen, Jeff Beck, FZ...
--sax
Michael Brecker(huh whish him to get better), John Zorn, Evan Parker, Igor Butman...
--piano
Jasper Van't Hoff, Chick Corea, Sergei Kuriokhin, Sasi Shalom, Pablo Ziglier...
--bass
Jeff Berlin, Percy Jones, Tony Levin
--drum(percussion)
Mike Portnoi, Bill Brufford, Larse Ulrich, Mark Nauseef, Nana Vasconcielos, Trilok Gurtu, Zakhir Hussain...

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.”

Newmanoc: Would you please enlighten us with a definition of each of the following terms in the context of the audiophile community:

Pace -
Rhythm -
Timing -

Thank you for your time.

John