Definitely try to write them as fast as possible, but IMO it's best to wrist as fast as possible, AND finish the essay, noting down the 30min mark, the 60min mark, the 90 min marks etc. (I actually used to mark down how long each para took me). This might help you target specific areas. If it takes you 30 minutes to write down 150 words, but you do the next 300 words in the next 30 minutes, clearly you struggle with introductions and should modify your intros or practice them. I also think stopping after an hour, especially if it's only 300 words is relatively useless. If you do as much as you can as fast as you can, at least you can get feedback on your essay as well as practise the timing. Just try to write in less time every time you write. (I'm assuming you haven't written much throughout the year, given your work situation etc? Honestly, it might just be an 'out of practice' thing... I tried to write a ULP essay yesterday along with the kids I tutor as a morale thing, and it was a lot harder to write in <60 minutes than I remember)