Before trying to write faster, make your structure predictable. Know how you will open, develop and close a response so that exam time is spent on ideas and language rather than deciding what to do next.
Plan briefly
Use a short outline before writing. Decide the purpose, two or three supporting ideas and the vocabulary you want to reuse.
Leave time to check
A final review for verb forms, agreement, spelling and connectors can protect points that are easy to lose.