I have never had a problem completing in the time specified; it’s been a DOMS issue for me. I started again recently and had to start at bodyweight with a low # of sets to avoid crippling DOMS the next several days.
I agree that keeping one leg on the box is faster, but you must be careful not to push off with the moving leg. I have also found that a broomstick for stabilization helps during the first few workouts.