I´m assuming you mean Jackson´s "Speed Neck", as most other manufacturers have other names for their profiles.... But please stop me if I´m barking up the wrong tree
The Jackson Speed profile is a thin, hard D, similar to a classical in general feel (some refer to it as "slightly blocky", becasue many prefer a more rounded contour. IMO ideally suited for shredders or players with a classical background, these necks provide a very "used to" feel for most classical players, while still coercing you to keep your hand in a classical position if you´re not from that background.....
On the other hand, if you´re more of a hendrix or Jeff beck style "Hand around the whole neck" fretter, you´ll almost certainly hate the profile as you´ll likely find it tiring quickly, and may want to rather consider the more "C-ish" profile of a Rhoads or Soloist
Specs: The speed profile is .775" @ 3rd fret tapering to .835 @ the 12th, again with a "hard D" feel.