What kind of photos are these, RAW or JPG or TIF or ??? What is the size of these images in pixels (width and height)?
RAW, 6000 x 4000 pixels from Nikon D7100.
I'm on Windows 7 with LR 2015.1.1 with no full-screen issues. Try disabling 'Use Graphics Processor' in LR Preferences> Performance tab. If this does not resolve the issue tell us what monitor(s) you are using and if the image is unsharp in both the Library and Develop modules or just one of them.
I have previously tried disabling the use graphics processor, (and various preview size & quality settings, returning to Auto 1366pix & High), with no improvement, and as this problem was first reported in version 5, pre-dating the v6 processor influence, I doubt that is the issue. It does seem better when you press F for full screen, then click on screen to zoom in. The flag 'loading' comes up, presumably generating a 1:1 preview, then when you zoom out again the full screen image looks better. If this is the culprit, why not generate the 1:1 immediately when entering full screen?
Anyway, not sure if that is it... but will monitor the situation over the next few days.
Even with this being flagged in LR5, I personally can't remember any issues with LR v5.7.