Assuming the technique is being done correctly, i.e., striking with flesh and nail, pushing towards the top of the guitar, motion driven from the first knuckle, efficient nail shape, relaxed hand and wrists, etc., then arguably the biggest limiting factor in picado development will be endurance. An easy way to demonstrate this can be done without the guitar. Use your hands and drum (on your chest, thighs, wherever) a single group of 16th notes (1 e & a 1) as fast as you evenly can. Now do four groups at that speed. Unless you happen to be a practiced drummer or percussionist, I would guess that you were not able to maintain accuracy and/or tempo. A simple way to improve this is a basic drum exercise. Starting at a comfortable tempo, add increasing groups of 16th notes over an 8th note rhythm. Applied to the guitar that would look something like this.
This should be done with a metronome. HERE is one on YouTube with practically every speed you’d need. Doing this for even just 5 to 10 minutes a day and gradually increasing the tempo should be enough to progress your picado endurance. As a bonus, the ability to fluently change between different note values will further develop your control.
Some additional notes:
You may find this more beneficial if you omit the non-bursting notes. This will provide increasingly shorter rest periods in between.
Practice starting with both fingers ( imim… and mimi…).
Practice different note lengths. E.g., 8th note triplet bursts over 8th note or 16th note triplets over 8th note triplets.
Change strings! Try doing so both before or after the group of 16ths or triplets. E.g., move down a string on each repeat of each example.
If you are tensing up, decrease the speed and focus on staying relaxed.
Try using the principles of this exercise with other techniques where you’d like to increase your endurance and control. HERE is this idea applied to alzapúa.