Have you ever tried timing your sets? For best results and for changing body shape sets should last between 40 to 70 seconds on most exercises. This will give your muscles enough time under tension to create a response from the exercise. If you are currently performing anywhere up to 15 repetitions per set and its only taking you 30 seconds or less than you are reducing intensity.
It’s difficult to maintain a consistent speed if too much weight is being used. Just lifting a weight from A to B without any regard to cadence will rob you of results and increase the risk of injury. Performing a set as quickly as possible isn’t the answer for most people’s goals. Training with a little less weight allows for more control and a consistent tempo throughout each set. For example, try starting with 5 seconds per repetition, a set of 8 repetitions should last 40 seconds.
This will be harder than doing double the amount of reps in less time. Give it a try; you’ll be amazed at the difference it makes.