Rhythm is the ratio and tempo is the speed.
Good putting strokes often have a ratio of 2:1. Again, it's the ratio of the putting stroke. You can have a 300ms backswing or a 600ms backswing, each with a 150 or a 300ms downswing, and that's 2:1. Both strokes have the same rhythm.
The tempo is the speed of the putter head. Short putts and long putts should have close to the same time (which is why, for example, I like to have a 78 BPM putting stroke), but will hav every different tempos. The shorter putt will have a slower tempo than the longer putt.