If you are writing to earn an income, you do need to know how to write quickly without losing the quality of your writing. The best way to write an article is to choose a topic that you already know something about. This will make the article easier to write.
Decide what angle you are going to use and create an outline. This will keep you from getting off topic. Quickly research each point of your outline and make notes. Set a timer so that you do not get distracted by reading something interesting but not relevant.
Once you have your information, sit down and write. Don't worry about spelling or grammar, just write. After you finish the article, you will go back over it and edit it. Look for first for spelling and grammar issues. Read it a second time to look for word flow and readability. With this method, I can usually write a 500 word article in about 30 minutes.