Developing left hand speed is way more difficult and takes way longer than picking speed. Anyone who has been playing for a while should know this by now.
When you play an alternate picking pattern, for example, if you are playing 3 notes per string, that’s actually 18 notes, but you’re only doing 6 string changes aka slanting. If you play 4 notes, is even more total notes, but still only 6 changes
Alternate picking in one string is very easy and is not the string changes holding you back, is the left hand not being able to keep up.
Thats what takes many and many years of practice and there is no trick or course that will change this.
Some people have an easier time gaining left hand speed, you can call it talent, genetics, etc but is a reality and it exists.
Similarly, some people despite countless pracgice may never achieved shred speeds on left hand either
The idea that anyone can achieve 200+bpm with just enough metronome and dedication is very naive, we all have limitations imposed by our physiology
You can become as fast as you can become tho, you won’t know until you try.
Its that simple. Exceptionalism at something is not meant for everyone if it was just practice and hard world, anyone could be tyson, phelps, messi, usain bolt, etc