Sweep-Picking.org

You are talking about classical – nylon string guitar, yes?
I can’t really see how playing with your finger nails versus your finger flesh has anything to do with speed. However, it has a lot to do with sound, and I haven’t seen or heard anyone that relies on just using the fingers – no nails.
Mind you, I can be wrong of course It’s just that the use of the nails is such an integral part of classical guitar – after all that’s the "pick".
For example, you can see in this video: http://www.metacafe.com/watch/262179/martha_masters_classical_guitar_jam_part_1/ how the nails on the artists right hand is very long (where the thumb nails act almost as a pick), while the nails on the fretting hand is short.
Or here: http://www.youtube.com/watch?v=JIiaY_MScUc – observe the different in finger nail length on the right versus the left (fretting) hand – it would be very hard to get this kind of things pulled off without having the finger nails trimmed correctly.

I can play pretty fast with a pick. Top speed is more like it. Don’t forget that even though we are only using a pick, we staill have the left hand to do pull-off and hammer-ons and it will sound like picking.