After hundreds of testing and published articles, especially on Wordpress blogs, I've come to the conclusion that it's for the best to NOT use the main keyword of an article in the URL slug.
Or at least, don't use it exactly as it is. Give it another form that still haves searches or use a synonym. On pages that I used a synonym for the main keywords in the URL slug did better than the ones with exact match type of keyword.
If you are using your main keyword in the title, meta description, H1 and a couple of times inside the content, I think Google sees it as over optimization if you also put it in the URL as exacted match.