In the examination setting where you have to write your essay to a certain word limit (say, 4000 words) and you have to do so against the clock, you will need plenty of writing stamina. That comes with practice.
While thinking and writing are deeply intermeshed activities, research tells us that the mind can focus well on a single activity at a time. That means the mechanics of good handwriting should be automated with practice.
Gradually lengthen your time trial practice sessions to achieve the desirable level of legibility, regularity and consistency of writing style, while your mental focus is 100% on the thinking process for the duration of the actual examination time limit.
When you are writing, your hand, arm and shoulder become a sophisticated orchestra of dozens of bones and muscles. Good writing technique redirects the effort of fingers and the wrist to the forearm and the shoulder. A good posture—back support, feet on the ground and knees at about 90˚ with plenty of leg room—facilitates the efficiency of the writing hand.