Swimming at the optimal speed

For distance swimmers one physiological factor is key: your lactate threshold. If you improve your lactate threshold pace, you will swim the same distance with less time. When training, your ability to sprint or work anaerobically above threshold is quite irrelevant when it comes to swimming long distances.

In order to improve your lactate threshold you should swim sets at your current threshold pace or just below it. Do not make the mistake of training above lactate threshold in short sharp swim sets - that isn't nearly as effective.

Source: Swim Smooth