I’d like to add, Paul Gilbert’s experience that his left hand was developed far before his right does not seem to be universal. Andy Wood in particular has mentioned on several of his streams (and I believe in one of his interviews here) that he has never had a problem with alternate picking, but when he picked up electric guitar he had a terrible time getting his legato working. While I don’t doubt that having good, in time, clean legato technique can help with alternate picking, it is certainly not a prerequisite.
Also, to the point I think @Troy was making, from a raw speed perspective, I suspect just about everyone can tap their fingers on a table faster than is necessary for almost any musical scenario. Just as a very non-scientific test, I set my metronome to 200 and tapped all 4 fingers in my left hand in a row to give me 16th notes. Ignoring each finger being in time individually and just focusing on hitting the beat each time, I could do it without even thinking about it. No trouble whatsoever. And I am by no means a great legato player. But since I am a guitar player, I put my wife to the test, who is not. With about 30 seconds to get the hang of timing it, she could easily do it too.
The way I think about it, that is one tap per finger per beat. Even way up at 300bpm sixteenths, that isn’t hard in the slightest. Anyone can do one tap per beat with any of their fingers at those speeds.
With all of that said and my little non-scientific tests complete, I’ve come to the same conclusion as Troy: the issue is in the accuracy and coordination, not in the speed.