Developing speed on a single string. Why is this lick so hard for me?

In the last couple of weeks I try to break through my own speed plateau. This lick seems so easy. But I still cannot manage it to play at 140 bpm with sixteenth notes.

The lick consists of three repeated finger movements^[1]:

1 - 4 - 3 - 4
1 - 4 - 2 - 4
1 - 4 - 2 (or 3) - 4

This the idea in E minor on the b string.

The first bar works, then it falls apart. After some research I found the concept of speed bursts mention online, in some forums and in some YouTube tutorials.

So I broke the lick rhythmically apart. On bar eigth notes and then the speed burst, one bar sixteenth notes. Although it is a quite long burst, you might ask. Indeed. Let's consider it a longer speed burst with endurance fix. The first bar with the the eight notes is perfectly suited to relax the left hand a bit. This works surprisingly very well. Be it the connection of the single finger movements with position shifts or the overall endurance.

In the next step. I reduced the eight notes one more to help endurance with the repeating sixteenth licks.

I am curious how long it will take with this approach to finally break my personal speed plateau.