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When you control your tempo, it is very important to keep your gliding time as long as possible.  To do so, I recommend to use a bit slower(by 0.1-0.2 sec) tempo first to build a correct form. Then change the tempo a bit faster(by 0.01-0.03 sec) and try to keep the same stroke count. In a few weeks, your base tempo will be faster with the same stroke count, which means you can swim faster.