How to quickly(but efffectively) do a Khan Academy course? I'm preparing for a upcoming math exam at the end of January.

This is probably too late to be of any help to you, so sorry about that, but if other people have a similar question, I hope this helps them! 

Here are some tips on the fastest way to finish a Khan Academy course: you should start by taking the Course Challenge to see what you need to review, then go into the units that you didn't get as high scores in (you may have to take the Course Challenge multiple times in order to test all the units) and take the Unit Test. If you don't understand a certain concept, use the videos, articles, exercises, and quizzes to review it. (To answer your question about the articles, watching videos and reading articles don't give you Mastery points, but they can help you understand the concept better, which is more important.) When you feel like you can do all the problems in the Unit Test and/or you are getting them mostly right, move on to the next unit.

I hope that you did well on your exam and that this helps anyone looking for similar info!

For any future users wanting to kind of full clear Kindergarten to Grade 8 Courses and beyond+ (so 100%) them, what I'd recommend is to do the "Quizzes" in each unit and then move on to the "Unit Test". You'd have to do the unit test 2-4+ times to get "Mastery" on specific topics. But if you do the Quiz -> Unit Test method, you get to become "Proficient" in the topics both literally (like you get to go through specific parts of the Unit to make sure you understand the Math) and the grading on Khan Academy.

After you get "Proficiency" in topics, you move on to the Unit Test as it recommends, and then you can get full-clear the unit. If any faster/more efficient methods are found by me, I'll update this.

But doing Unit Tests and Course Challenges aren't as effective at full-clearing. They are good at identifying weaknesses in your knowledge and understanding fast though.

I need to get to 90 by Mar. 31 and I am really struggling with leveling up - it takes forever once you get past 70% :((((