Give at least 10 minutes speed test daily. Write as fast as you can. Whatever came to your mind just focus on only speed, not ideas and beautiful writing. If we do something daily it can give you surprising results.
Next, do some creativity. Make situation tougher for writing. Write with the middle finger and thumb, little finger and thumb, with very rough paper, with a different pen, with a fountain pen. Try with left-hand writing. Any other creative idea came into your mind. After this when you will write with favourable conditions you can get even more improved results.
And last practice for whole 3 hours daily about one month and if possible chose possible real exam time like your exam may be 12 to 3 pm. Write one month 12 to 3 pm.
And finally also work on thinking and idea generation speed, develop your understanding speed, analysing speed, recall speed, grasping speed so that it can match with your writing speed.
Please share with me if you willing to apply and find positive results.
These ideas are just logically generated and many are not tested before this by anyone.