What does 'Fast mean?

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.

Newmanoc: actually I think you're right in that "pratt" probably originated with Naim when the two Cos were working closely together. THEN the Linn marketing/sales people did quite a lot of use of that with their dealers. It;s gone out of fashion now I suppose...

Loosely defined, the erstwhile "pratt" meant a "crispy" sound in the all important 100-10kHz region with close focus on 300-~6kHz: the point was to get FR and phase as good as possible in that range (think of the old LP12).

Much of the music listened to & used for auditioning at the time was rythmic (4/4, 8/4 etc) with vocals. Sooo, if you focused on the main part of the frequency spectrum rather than the extensions -- i.e. your 300Hz blended in well with the 5kHz say -- you had this lively, pleasant sound. I.e. "foot-tapping" sound as the good Mr Tiefenbrun used to say.
Focus on the range & its slight prominence also meant that with speakers reasonably (but not painstakingly) set up, the "springy" effect was still there.
Voila in reductionist and simplified form. Linn & Naim can offer a much more precise & sophisticated explanation, of course. Cheers