I think Julie and Nick hit the nail on the head. Fionn, if you read my note to you, don't mistake my answer to your question on dance tempo as a standard for sessions. It was only an answer to your specific question. For sessions,my first entry was a way of saying that the main thing is to get into a groove. The "groove" may be found at various speeds, and it's a special experience when it happens, but you just know when it's clicking and your comfortable with it. When you're in a groove, you can play at a fast clip without sounding rushed or slowly without dragging the tune. "How fast is fast and how slow is slow" is not, then, a matter of specific metronome tempi, but of feeling your way into that groove. Make sense?