Breaststroke speed is greatly influenced from the kick. Breaststroke kick utilizes transverse hip motion (internal and external rotation) opposed to sagittal plane motion (flexion and extension) in the other three strokes. Specifically, breaststroke requires a large amount of hip internal rotation, helping the swimmer outsweep their feet and have a greater force production capacity. If you have difficulties with your breaststroke kick, try this self myofascial release (SMR) and if you need more tips, check out Mobility for Swimmers (http://www.swimmingscience.net/2014/01/mobilityforswimmers.html).