Rather, prepare to race. Any physiological benefits from warming up disappear within 20 minutes. If you cannot connect the warm up with your race, the only benefit is a psychological one (plus possibly familiarising yourself with the pool). Turn your ‘warm up’ into a focussed Race Preparation session. Get yourself into a mental place where you leave the water ready to win!

The easiest way is to reduce your resistance as much as possible at all times when you are swimming

Those changes actually take place between training sessions - and only if you give your body a proper chance to allow them to happen. Rest and recovery is key here. Get enough sleep and try to avoid becoming over tired.