Lady_Liah has some really good advice, and once you do know what you want to write for the chapter, there are some tricks you can do to speed up the process of physically writing/typing (as in you have a scene in your mind but can't get the words down).
I will start by saying that I have a problem of re-reading my previous sentences during the writing process (at least fifteen times for every sentence). This actually takes me out of the gear of writing and into the gear of editing because I also always fiddle with them. This slows my writing process a lot. It is also superfluous because when I read my stuff the following day, I'm going to change it anyhow. So, what to do?
Write and let those ugly first sentences stay for a while. Once you have written what you had wanted to write, proofread it to make sure you wrote down what you meant to say (sometimes I accidentally leave out an important word, which might be my unique typing problem). Then, take a break, or better yet, come back to it another day (the longer the better). With fresh eyes, edit.
This has helped me tremendously (idk if this will work for you since it's under the assumption you edit as you write).
Here's an exercise that might help:
First close your eyes then start typing. Type whatever words come to your mind when you are forming your scenes and sentences. If you make spelling mistakes, that's ok. It's still understandable. If you write half a sentence then forget what you write, just enter some blank spaces and begin the next (or same) sentence starting with a capital letter. Later you can clear up the mistakes. If you find yourself opening your eyes to read the sentences, you can also cover the screen/monitor with a piece of paper or light fabric. If you forget your place in the scene, peeking to jog your memory is perfectly fine.
Then edit the crap out of it so that it's understandable. Remove the parts you don't like, expand on the ones you love (or make a note of what you want to expand on), and shuffle the sentence/scene orderings so that it roughly flows properly.
The result should be a rough draft in need of actual editing (for another day or after a long break)
For me, this doubles the speed at which I write/type, but at the sacrifice of making the initial editing take more time (by initial editing, I mean the editing that creates a first draft, not the editing to polish the first draft).
FYI: I don't do this all the time. I mainly do it to stop myself from re-reading previous sentences when I discover myself slipping back into my old habit. I also find it pretty mentally draining once I go for a while. I think the longest I have ever done it for was about two to three hours (this was without break; also my actual attention span is ridiculously long so I don't expect any sane, normal person to go longer than an hour).