Once you decide to do a certain number of reps it's natural to want to complete those reps as quickly and easily as possible. It's our body's default to move things efficiently, which means it always wants to get from Point A to B with minimal work. Unless controlled for, that default eliminates the exercise's value.
The focus should always be on the feeling the muscles contract and to resist the urge to "bounce" or "launch" weights around. A set that lasts 60-90 seconds while only doing 2-3 reps has much more value than 10-20 mindless movements. Stop counting and start focusing on what matters most - CONTRACTING MUSCLES!