In Lightroom 6/CC, there is a new "Quality" setting for slideshows, at the very bottom of the "Playback" section of the settings (at the bottom of the right-hand sidebar in the Slideshow module). It has three settings: low / standard / high .
The new setting defaults to "Standard," which on a decent resolution monitor results in blurriness. The "High" setting is equivalent to how Lightroom 5 used to display slideshows.
So, try changing it to "High" and see if that fixes the problem for you.
Unfortunately, for me, the Slideshow Quality setting resets to "Standard" every time I quit Lightroom 6.0, when I want it to stay on "High." Very annoying.
I filed a problem report about it here:
My slide show doesn't work at all what is going on
I noticed the same unsharpness in the slideshow module, I find this incredible to say the least! How difficult can it be to show an image simply pixel perfect (at least when shown at 100%)?
I did a test with a 1920x1080 checkerboard image, and indeed: it is unable to simply show this checkerboard image: parts of it are perfect, but other parts are changed to grey. First I had to revert to an old version of lightroom because of navigation problems, and now that was solved we're stuck with this problem.
Anyway: I don't only want to complain: Photoshop is becoming better and better, I mean...how could I ever do without 'content-aware fill' again....?