I think it's difficult to say which requires the most leg speed as the movement required for serving and volleying is different from moving from side to side at the baseline.
Borg was incredibly quick at the baseline, but so was Sampras all over the court. There's nver been a better mover than Federer, or someone as capable of getting balls back into play from almost impossible positions than Nadal (and Djokovic recently)
Due to the changed nature of the game, i.e. all-court rather than serve/volley, chip and charge etc it requires different sorts of athleticism.
And the thing is that speed around the court is only part of the picture. Many "athletes" from other sports are incredibly quick, but put them on a tennis court and they don't move well at all. This is because anticipation an knowing where youir opponent is likely going to hit the ball are also part of what make players look quick.
Happy Valentines Day
THE TENNIS GUY