This is easiest to remember by referring back to the common goal--making music.
The FASTER the tempo, the greater the tendency of repeated notes (or of any notes of an articulate line) to run into each other. The breaks between notes must grow relatively LARGER if the various notes are to be heard independently of one another. The breaks can become as large as half the length of the prevailing note value in fast tempos.
The SLOWER the tempo, the greater the tendency of repeated notes (or of any notes of an articulate line) to separate from one another. The breaks between notes must grow relatively SMALLER if the various notes are to be heard cohesively as a musical line. The breaks can become a small fraction of the length of the prevailing note value in slow tempos.