I had a problem in LR4 and just upgraded to LR6, thinking it would fix the problem. I'm still having the same issue so there must be something I'm not understanding.
When I view a single photo in the library, either full screen or with 4 on the screen, the picture starts out blurry and takes a few seconds to sharpen while it loads. This happens even after I create 1:1 previews.
I'm on a Macbook Air with extra hard drive space and enough RAM.