Lightroom develop module Uses your Graphics Processor and displays the image after rendering.
Are you looking at the images at actual pixels/100% zoom?
The most accurate way to view your images is in Develop when set to 1:1 (or greater). Develop previews shows noise reduction and sharpening at this zoom ratio where Library module doesn't do that.
What happens if you disable Graphics Processor usage in Preferences >>Performance >>Uncheck "Use graphics Prosessor" ?
This is so random. I unchecked the box above. It seemed to fix it. I went back and rechecked it and it seems to remain fixed. I get that the most accurate way to view the photo is in Develop 1:1, but when you transfer back to the Library module, the photo should still look the same. It's almost like in the Library module it removed all of the sharpening.
See this topic in the official Adobe feedback forum: Lightroom: Low Quality Downscaled Views in the Develop Module – Part II. I believe it continues to be the most authoritative explanation of the issue, including for LR CC 2015 / 6. The basic issue is that LR uses a faster but less accurate algorithm for downsizing images in Loupe, and that this becomes most noticeable when you have high ISO images and a large amount of sharpening.
Also, best to continue any discussion in that other topic in the feedback forum -- others who are more authoritative (including Adobe product engineers) are more likley to see it.