Besides the two different types of playing, slow or fast. A very important concept is presentation.
Your body will adapt to the forces you put it under, this means to play fast you simply play faster and jump into those higher inertia forces to build up your muscles.
And gravity also plays a big role, if you practice sat down with the guitar in a position different from the guitars natural position when standing, your body will adapt to those forces, meaning soon as you sand to present your skill to others, you feel like you’re holding an alien, all the forces change.
So it’s very important to practice as if you’re presenting your guitar playing to an audience. So somehow try get yourself in a comfy position with the guitar near exactly how it hangs when standing.
It’s annoying as it takes some comfort away from hours of practice, but the way you hold the guitar when practicing can mean the difference between bedroom player and legit guitarist.