I think that 'speed' comes from numerous factors - the radius of the fretboard, the tension in the strings, the action of the strings, the shape and thickness of the neck, the position of the guitar (like the way it hangs and length of the strap), the gauge of the strings etc etc.
However, the one thing that matters in all of this is the musician, their comfort and what suits them best. There are some fast playing musicians that prefer a thicker neck, a shorter scale length, a more curved fretboard radius for example whilst another can only play fast if the neck is ridiculously thin with a very flat fretboard, thinner strings and a longer scale length.
What may suit me for playing 'fast' may not be what suits you. Admittedly, a lot of players known for speed do tend to gravitate to the skinnier necks and flatter radius fretboards - something like an Ibanez wizard neck for example - that is commonly associated with Shredders. Yngwie though plays a Strat with a typical strat neck, scalloped frets and I think the 7.25" fretboard Radius - its certainly not as flat as the 10" radius of a PRS. There are quite a few artists tat aren't slow that play PRS...
The best bet is to go to a store, try a Pattern Thin Neck and see if you feel most comfortable playing this. If you are not fast and are hoping to get 'fast', then find the neck that feels most comfortable to you because to get fast, you will need to practice, practice and practice. If you pick a guitar that isn't the most comfortable, then that will impact on how much practice you can do. Anything that can help you practice more without causing pain/discomfort etc the better.
Of course, if you are already fast, you must know that in which case, you will know that the guitar that feels the best to you is the guitar to buy for shredding...
