interesting enough I consider there to be two parts to the process...
one is the mental aspect or sight reading music, easily recognizing the patterns, etc. in each key signature...
and the other is the physical, and to me more fun, how nimble and strong are your fingers? running up and down the keyboard playing scales, and making up one's own pieces is most of the fun... The opening of Chopin's Revolutionary Prelude is deceptively simple, and yet it took me a month of practice for my hand to fly over it, and even now I only sound half decent... I suppose this is why you need to play the same thing over and over and over again (after it has been memorized) to get just the right intonation, etc.