There are very significant differences between fretting technique for slow and fast playing.
This is a deep topic, and I can’t explain everything here in this comment, but I’ll try to outline the big picture here.
When fretting slowly, it’s possible to control subtle movements of the individual fingers in ways that are not possible at speed. This level of control is achieved through careful balancing of opposing muscle groups. In single-note playing, these movements are often used for fretting hand string tracking and position tracking. In chordal playing, these movements are absolutely ubiquitous; every chord is essentially a contortion of the fretting hand.
However, fingers are not independent in any sense. The movement of any finger affects the movement of all others (to varying degrees). The type of control mentioned above requires constant activation of muscles that are common to all fingers. In particular, flexor digitorum profundus (FDP) and extensor digitorum communis (EDC). These muscles are in the forearm and are significantly stronger than the intrinsic muscles of the hand. The intrinsic muscles of the hand act against the communal muscles to achieve fine control at low speeds.
We simply can’t achieve this level of fine control when we play faster. The constant activation of FDP and EDC interferes with the smaller muscles. The fretting hand becomes rigid and the muscles fatigue quickly.
We can’t fix the problem by focusing on “finger independence” at slower speeds, because fingers are not independent and can not be trained to become independent. Trying to focus on “economy of motion” at slower speeds doesn’t work either, this just increases the level of cocontraction.
More than that, when playing slowly, it is possible (and often necessary) to fret with gradual pressure in a way that’s not possible at higher speed. At higher speed, fretting forces must be generated much more suddenly. Imagine the difference between “pressing” the string and “striking” the string.
When we play at speed, we have limited ability to “aim” the fingers individually. The fingers must closely follow their natural arc of motion (like a reflexive grasp). We can not “aim” the fingers, so we must “aim” the hand.
As an analogy, think of a WW2 fighter plane. The machine guns were fixed, so the pilots couldn’t aim the guns. Instead, they aimed the plane through aerial maneuvers.
The “aerial maneuvers” of the fretting hand are often not necessary when playing slowly. However, these maneuvers are absolutely essential in fast playing, so the fretting hand must be maneuverable.
The “maneuvers” required are characteristic to each pattern or lick. They are inherently rhythmic, connecting the fretting hand to the internal clock. This is critical to chunking and synchronisation.
Practicing too slowly allows for the type of fine finger control mentioned previously, and so does not reliably result in the emergence of these maneuvers. Practicing in a fashion that is non-representative of fast playing can actually impede their development.
Unfortunately, none of this is actionable until we deal with some more foundational issues. We have to accept the natural interdependence of our fingers – some fingerings are just inherently inefficient. We also have to address the reuse problem and develop strategies to minimize the frequencies of our movements. The efficient digital cycles are the generic “optimum,” but we can also utilize situational movements (the rock, the roll and the reveal) while fretting.
When all of this is understood, we can at least start with things that we can reasonably expect to be fast. If we can habituate fretting postures which are similar to the hand at rest, which naturally align our muscles to the task and which are maneuverable, then we can actually practice those things in a manner that promotes the emergence of the appropriate movement patterns.