"Don't get it right, get it written."
It's significantly easier to write something bad first and then edit it into greatness than it is to write something great the first time around. When you write badly on paper, you at least get to see it and realize why it doesn't work, whereas if you just sit there and think about all the ideas you have until the right words or ideas formulate, you'll be wasting time retreading ideas that should have been discarded long ago. Writing, like any other skill, requires trial and error before you get good at it. Knowing the fundamentals helps but it doesn't make you a flawless writer. Therefore, the faster you can fail, the faster you can improve. I know a lot of people out there wear the word "perfectionist" like a badge of honor, but make no mistake, perfectionism is a bad thing; No one does anything perfectly the first time around, so accept that fact and start failing fast!
That being said, you should get good at editing your writing. This is both a mindset and a skill. When editing you should look at your writing through an objective lens even though it is difficult when there's an emotional attachment to what you wrote. Cut out unnecessary and boring stuff, make exposition more fun or look for ways to integrate it into the images instead, and make sure what is being said makes sense. Grammar, spelling, and punctuations are a given.
I thought about explaining my writing process here, but then I've already procrastinated long enough just writing what I've written above. So I'll leave you with this: You writing slow might be a symptom that either you don't know where the story is going or you're too afraid to put pen to paper because it might not turn out as good as you'd like. If it's the former, make an outline of your story (I know everyone hates it, but do it, it helps), if it's the later, it'll require a mindset shift.
