2015.6 seems to have lots of performance issues that many are reporting. I've just noticed another which I'd be interested to hear if others experience too.
In Slideshow I have always just used the Standard quality option and the results have always been fine for my iMac screen (27" 2560x1440). Yesterday I created a slideshow for a few new collections I'm working on and again used the Standard quality setting but when they played the photos were pretty poor quality. They didn't look over compressed i.e. full of nasty overly compressed jpeg artifacts, they looked out of focus. The whole image looked slightly blurred and every image was the same. Regenerating the slideshow in high quality and the result was much better.
I recently upgraded my camera from an 8MP Canon 30D to a 22MP Canon 5D MKIII so in case it was a file resolution thing I went back to some older shots taken with my previous camera and these too exhibited the same problem, albeit not as severe, which it never used to do in earlier versions of LR.
If you're using 'Play' full-screen mode I would expect to see a difference between Standard and High Quality settings. Even on High setting in full-screen Play mode the image isn't that sharp. I'm using very similar Canon 5D MK II 21Mp raw files with a 2560x1440 monitor. I also tried Canon 300D 6Mp raw files and they appear slightly sharper. To see how the slideshow images should look create a PDF slideshow using the 'Screen' size setting. Open the PDF in Adobe Reader or Acrobat and then use CTRL+L keys to view it in full-screen mode.
I viewed the same 5D MKII raw files in LR5.7.1 slideshow full-screen Play mode and they are much sharper and very close to the PDF output quality. So clearly something happened, at some point, with the LR 6/CC 2015 Slideshow module. My guess is that output sharpening isn't being applied to the full-screen mode Slideshow images.
You can post a problem report here: Photoshop Family Customer Community