writing big wont usually help, because teachers and HSC markers arent stupid, if us students can figure out this 'trick', im sure they can as well.
the only way to get faster writing in my oppinion, is to keep a diary. every night spend only 10 minutes writing a diary entry, in these 10 minutes cram in as much bullshit as you can.
simply write about what happened in the day that passed, you'll never run out of shit to write
in yr 10, i had the same problem as u did, lots of info in my head, all structured and ready to go, but i wrote about 1 page in 45 minutes.
then i kept that diary, 10 minutes a night isn't that hard to find. i got out a piece of scrap paper, and WROTE for 10 minutes.
i did it for about 2 weeks and already my writing had improved dramatically.
nowadays, i can write fairly quickly