Piano, Organ, and Keyboard/slow playing
One often think that playing fast is the most difficult thing in music but I am not sure. Slow playing can be even more difficult sometimes. Is this something you've experienced to? With fast tempo you can go by rhythmic feel but with slower tempos you may even count the rhythms. What are your expert views on this?
Determining what is more difficult between slower and faster speeds is a matter of perspective and depends on the context of the music.
Going by rhythmic feel is a broad concept that can apply to either a slow or fast tempo.
Going by exactness of rhythms relative to any given tempo is a very specific approach, which could allow for no error if taken to the extreme.
Consider the music itself. Is it meant to be performed with a rigid feel and exactness? That can be accomplished by subdividing everything and not allowing for any push or pull. However, that does not mean that you have to omit expression entirely, as there are still other elements to the music, such as dynamics.
Even the timing can be portrayed with push and pull if the rhythms are written in a way that make the music appear as though it is speeding up or slowing down.
Generally speaking, if the music contains rhythms that are very complex, that might allow for an easier performance, regardless of tempo, as long as you master the rhythms. However, it might not. Slower could be easier or harder. Faster could be easier or harder. It goes back to the context of the music itself.
The same idea applies, generally speaking, if the music contains very simplistic rhythms.
Does the composer intend for their music to be performed in a certain way? How much freedom do they want the performer to have with it? It might be easier to perform music with more sustained or more spaced-out notes at a faster tempo, and easier to perform music with more active changes among their rhythms at a slower tempo, although it ultimately depends on the context of the music, including all of the musical elements.
Best of luck!