Never sacrifice form with speed, performing 5 perfect press-ups is exactly the same as performing 20 sloppy rushed press-ups. By concentrating on form in each exercise you are maximizing the full potential of that exercise and getting the most out of your target muscle groups, but as you start to lose form you start targeting the wrong muscles and you start to lose the benefit of that exercise. So if you feel that you are losing form either slow down your exercise or stop take a breath, concentrate and then carrying on.