Thanks for reporting, but I am not sure where exactly we should look. Is there some performance issue? Can you try rendering the same scene with the same region in Corona 6 and 7 and see if there is some difference?
In your current screenshots it looks like you are comparing two different scenes, so I am not sure how to understand this.
Also, are you using some kind of render limit? If so, what limit is it? (pass, time, or noise limit?)
Lastly, it looks like you uploaded the same image twice (unless I can't spot the difference).