how can I speed up my writing in exams

By practicing more and more. Practicing will not only improve your mental calculations, but also will enable you to just know how to solve a each and every type of question that can possibly be asked in the exam, due to which you wouldn't be stuck thinking about any question in the exam. 

 

Also, make sure that you know all the tables from 1 to 20, their squares and square roots, cubes and cube root by heart. This will help you a lot.