What does 'Fast mean?

Boa2, that does sound fast. You got anything faster? In regards to this hobby I do, how about a vacuum cleaner set to maximum suction hooked up to your wallet. Now that is fast! If you hear a sucking sound, you know where I've been. If anybody finds a wallet with my initials, please at least return the credit cards & drivers license, you keep the wallet & what little money if any, remains.

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.

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.