There exist several different motion mechanics for alternate picking, all of which can be very effective and all of which have been used by guitarists with elite level picking speed. For the purposes of this post I’ll use the wrist motion mechanic as an example to illustrate my point.
Consider wrist deviation for example. It describes moving the wrist back and forth in a certain way. Any healthy person can move his wrist back and forth with wrist deviation ( or wrist flexion and extension if you prefer to use that as an example), but there is a limit to the speed he will be able to reach at which point, no greater speed will be possible. Sufficient practice of the right kind will result in a guitarist being able to move his wrist back and forth faster than when he first started practicing. If you were to take a hundred beginner guitarists who all used the same wrist motion mechanic, and they all practiced in an efficient way for a sufficient amount of time they would all improve in speed until they reach a point at which it would become impossible for them to generate any additional picking speed using that motion mechanic. Why is it then, one might ask, are some of them able to generate so much more picking speed than others if they all practiced in a highly efficient manner for a sufficient amount of time?
The answer is they all had a certain genetic potential for the amount of speed with which they were able to move their wrists back and forth. Along the 100 guitarists in that group you would find that their maximum potential picking speed would lie along a continuum that would range from fairly slow to very fast. You could train 100 men to be sprinters and again, with a sufficient amount of practice of the right kind, they would all reach a speed at which they would find further progress to be impossible, i.e. their genetic potential would have been reached.
The only way to find out what your genetic potential is, is to use one of the accepted motion mechanics for high speed picking and then practice your picking technique in an efficient manner for long enough until you are unable to develop any additional speed. Your “speed limit” might be 16th notes at 140 beats a minute or it might be 16th notes at 240 beats a minute. What the cause is for there to be such a wide range in top speeds from one end of the continuum to the other could probably be best explained by a competent exercise physiologist or perhaps a neurologist. Basically, it is at least partially dictated by the top speed at which neurological impulses are sent and able to travel along your central nervous system and also by the ability the muscles in your hands, wrists, and forearms to carry out these orders sent out by the brain when they receive the neurological impulses sent. No matter how fast your central nervous system is able to send commands to your muscles in the form of neurological impulses, your muscles still have to have the ability to carry out these commands.
Just as people’s central nervous systems vary with the speed at which they are able to send impulses to their muscles, their muscles also vary with regard to how fast they are able to contract and relax. That’s all a voluntary muscle can do: contract and relax. They create muscular force. That’s what they’re for and that’s all they’re designed to do, Different people will vary in relation to the number of muscle fibers they have within a particular voluntary muscle, the types of muscle fibers present in those muscles, and the ability of those muscles to grow in size and strength which is what affects the speed at which they can, in this instance, move your guitar pick.
When you have reached your genetic potential regarding the development in your ability to move a guitar pick back and forth, you will have reached a level of speed at which you cannot develop further picking speed. That may end up being dictated by having reached your central nervous system’s limit in it’s ability to increase the speed at which it can send neurological impulses to the muscles responsible for moving your pick back and forth. It may be that while your neurological limits have not been reached, the muscles which are responsible for you being able to move your pick rapidly have been developed as far as your genetics will allow. I doubt you’ll ever be able to find someone who can verify which of those factors resulted in further progress in picking speed for you to be impossible and it probably shouldn’t matter to you. Whatever the cause may be, you’ve then reached your limit for developing greater picking speed.