Have you ever felt like you were spinning in one place when you were trying to sprint? No matter how hard you work, you feel that you're not moving very fast? One of the reasons why this happens is because your rhythm and timing are off, a lot of swimmers have moments like this.
When your rhythm is off, you lose speed between strokes because you are speeding up and slowing down. This start/stop effect is very exhausting. Using a constant rhythm is more efficient, taking less energy to sustain your speed. Rhythm, however, is very difficult for some swimmer to feel. If you cannot feel it, it's hard to fix. (It's often hard to even identify it as a problem since it just feels like you are expending a lot of energy without producing a lot of speed or distance.)
I know this from personal experience. I have often had difficulties finding a consistent rhythm until a few months ago, after I had been training with DragSox for about 6 weeks. As I have mentioned in previous posts, DragSox slow you down so that you feel everything. It's as though they strip down swimming to its bare essentials so you can concentrate on what's absolutely essential. Swimming with DragSox, I realized pretty quickly that I was wasting energy and that my rhythm was off. I started to make very small adjustments and noticed differences immediately. Focusing on these incremental changes made an overall impact to my rhythm. I became a more efficient swimmer because I was able to identify the underlying problem and feel the impact of the small change to my technique. Now that I have a more consistent rhythm, I don't get nearly as tired and I am swimming faster.
Here are some drills that helped me: head up freestyle, dolphin kick freestyle and wide kick on your side (keep your legs straight and try to kick as wide as you can; one arm is extended in front of you).
Be sure to slow down and really feel what you are doing. Take note of all the little changes you make and you'll discover what works best for you.