There are two main weight loss methods which will an an adverse effect on your peformance, these are dehydration and crash dieting.
There is very little evidence to prove that a lower body fat/weight will improve your performance. The best way to improve performance is via good nutrition and strenuous training.
In fact losing weight quickly can diminish your aerobic capacity. Atheletes who have lost 2-3 % body weight through dehydration can have a 5% decrease in their aerobic capacity. Where as those who lose weight via strict dieting can lose up to 10% aerobic capacity.
So, DON’T crash diet!