Oh you poor thing, I had this problem years ago, it's so frustrating isn't it!
My solution was to just spend 10 mins on a plan and 50 mins on the essay, and keep strictly to it, stopping after an hour even if I didn't finish ... even if I had nothing in my head I would just make some crap up which hopefully had some relevance to the topic.
Initially those essays were absolute garbage and were only about 400 words long. But I just kept doing it and gradually I could see word length and quality increasing (word count increasing far more rapidly than quality unfortunately). Slowly you'll train your brain to think quickly on the spot.
Having a plan actually really helped me to write the essay faster because all the thinking was already done in the plan and all I had to do was just write it out in longhand.
(this post would have gone up faster but marbs' display pic hypnotised me. seriously I was sitting here staring at the THATS RACIST kid for like 5 mins

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