Dexterity
I was noticing that I struggle when playing piano at fast tempos. This, of course, is completely natural and very common, because it requires a lot of finesse to press small keys quickly while moving the hands laterally along a keyboard. I've studied a lot of harmony and (lately) melodic shapes, which I don't struggle with as much. However, in practice, the physical limits of my hands has been slowing down my ability to convey the ideas I hear in my head. This, most likely, is due to a lack of practicing stamina and mobility.

I used to think that practicing quickly for the sake of playing quickly was a sort of showy, vapid strategy. I've heard many musicians say that showiness and dazzling technique are nothing next to beautiful and moving content. It's the material that matters.

While that is true, it is also important to keep your hands in shape (musically speaking). I think this means practicing, to some extent, fast passages that repeat dozens of times, that tire out and ultimately strengthen your hands. I think I'll start with Schmitt, Op. 16, the Preparatory Exercises.

Like all things, balance is crucial.