Hi Dan. Thank you for the comment. I've done something similar (on exercises that allow it) taking 30 seconds each on the lowering and lifting portion of an exercise, it's a killer! Exercises where you end in the bottom position allow the final 30 seconds to be a negative (lowering) and are especially brutal /effective. I managed to do that with squats by setting the safety pins at the right height.
Another routine i enjoyed was picking two ezercises and supersetting them for 7 minutes non stop, so spending 3.5 minutes on each one. Can't remember where i got that routine from but it was enjoyable.
I think time under tension may well be a better metric than reps. And slower exrcise cadence promotes better form, improved flexibility and better results.