People tend to speak about 120 to 160 words per minute depending on the context. Typing 50 to 60 words per minute is a comfortable pace.
That means the only thing holding back writing is the speed of typing. It shouldn’t be the speed of thought. Write like you speak and try to speak slowly in your mind so your fingers can keep up and you should be able to communicate any message, in written form, without much effort.
The only reason you might write slower than you speak is because you aren’t thinking conversationally.