It's interesting that the article says slowing down makes the pushup harder. That logically makes sense to me. But when I'm mid workout, and I can't keep up with Cathe going "at tempo", I'll usually do sets of 2/2 instead of singles. So if Cathe does 16 singles, I'll do 8 in 2/2 pace.
I find I can manage my form much better slowing it down. I've never considered that I might be working harder that way! Or maybe that doesn't count and I should be doing 16 at 2/2 pace...