No, there is no evidence on having a strong core. You are right. But there is research about about speech rate and one of the techniques that is used is stretched syllable technique. When stretching our syllables we have to control how much air is coming out from our lungs. The air that comes out from our diaphragm it’s one of the main sources of speech. Having control of how we breathe makes it easier to control how that air comes out. Do I want to have that big puff of air that keeps me stuck on the sounds or do we want to let that air come out slowly so we can stretch out those sounds and finish our sentence?