how can people alternate pick so fast?

[rich2k4]

i'm talking about on 1 string and just alternate picking an open string.

i try to do it, and i can't get fast at all. but my friend who doesn't even practice speed can get it going pretty fast.

it makes it seem like some people are able to play fast, and others, no matter how much they practice or try will never be able to shred/play fast etc.

i've been returning to the speed thing every once in a while. it's something i want to accomplish as a goal even though i know it's not musical if used wrong. but it's just something i've always wanted to do. stuff like guthrie govan.

i've been working at it for the past 2 years and it seems like no matter how much i practice, even with a metronome etc. i just can't do it. there is always some kind of barrier i hit where i can't get any faster. no matter how long i sit down and try to find what i am doing wrong, and no matter how long i spend playing at 1 particular speed. i can never pass this certain barrier.