I think this problem exists since Lightroom 4 (or longer). The previews are not sharp. Full screen mode has a slight blur too. Now comes the funny part.
When I click the loupe and HOLD THE CLICK, it shows the sharp image. If I click WITHOUT holding the mouse button, it zooms in (to the ratio I have set), but the image is slightly blurry. Interestingly for a few milliseconds after clicking (without holding), it shows the sharp image, but goes aways right after. So, I have to click and hold to see the sharp image and check if it is sharp enough.
Well, at least I have found a workaround for me, but I wonder why Adobe doesn't manage to fix this...
Anyway, it might help a few guys out there who experience the same problem with unsharp preivews.