I don't know the answer but I will throw out some thoughts that may trigger something in another reader. First, according to the download version of your manual, page 8, it says you should have the following quality settings:
"Highest" is for precise image printing, using the highest resolutions, most ink and longest time to print each page. "Fast" is brother's way of saying draft quality - light grays instead of deep blacks, using the least amount of time and ink to print each page. Do you see the other settings?
What you describe as happening, however, sounds like pixelization - what you would see if printing a small digital image that has been blown up into a large image. If that is the case, then I expect you need to tell the printer to print in actual size.
Does text look good?