You remember the blue books teachers used to pass out for exams? You would crack one open when your instructor said, "Begin," and you wrote as fast as your mind could move the pencil. Your middle finger got a cupped indentation from the wooden shaft where it rested while you wrote hard. Barely thinking. Scribbling until your penmanship went awobble. You erased the indecipherable words, slowed down, and then wrote them again. Fast.
So, when you compose your first draft, just pretend that you're taking an essay exam. You have only until the end of the hour to finish. Your grade depends on it. You really have to finish because an incomplete blue book will make you look stupid.
The point is: Get that whole draft done from start to finish.
Unlike school, you have the luxury at your home desk of being able to polish it once it's done.
But you must finish. Now! The bell's going to ring!
You have only so much time to complete your answer.