The human brain needs 0.1s to get the visual image, think what to do, and send an order to the muscles. If you have full muscle memory, you don't think what are you doing when you are executing algorithms, so the only thing that matters is recognition time.
Using Fridrich as an example, the fastest way to solve a cube is using 6 algorithms, 4 for F2L and 2 for LL. Why using algs for F2L?. Well, if you do intuitive F2L, your times will be conditioned by your recognition (we are thinking what would happen in a perfectly fast solve).
So, nowadays, the fastest cubers can execute some algorithms in less than a second, more or less. Let's just say that our perfect cuber can do all of them all ~0.90s each. And the cross, he could make it in less than a second too right? let's give him an avg of 0.9s.
[6 algs*0.90s] + [6 algs*0.1s] + 0.9s = 6.9s
(So our perfect cuber is able to have 6.9s non-lucky solves with no effort using Fridrich)
If he has an easy scramble, xcross, OLL or PLL skips... damn.
All in all, I think we are reaching the 3x3x3 time limit for the Fridrich method at least.