Staying relaxed is one of the most valuable states have while speaking English. The oral musculature (tongue, lips, and jaw) vary in tension for the sounds of the English language, but general relaxation of the oral musculature is essential.
Our vocal cords pull together by way of the Bernoulli effect. (If you aren’t familiar with physics, that’s ok! This has to do with air pressure and the way our vocal folds pull together via a vacuum.) However, when the muscles around the larynx are tense, it affects the corresponding sound. The muscles of the larynx and the vocal tract need to be tension free to perform optimally. Imagine holding a tight fist for a period of time. When you are ready to pick up an object, the muscles of your hand will be stiff and tired for a while even for a simple act like picking up something.