Hi karas
Could you make sure that Picture are not missing in your Lightroom CC/6 catalog. You will see a "!" sign on top right of Image Thumbnail.See Adobe Photoshop Lightroom Help | Locate missing photos
If not so,this is Probably an issues with Corrupt Monitor Profile.Refer this link for solution Photos appear as black, gray rectangles | Lightroom | Windows
. When scrolling through the thumbnails, it moves so slowly that it is unusable.
make sure your Graphics card drivers are up to date.Disable Use of Graphics Processor and see if Lightroom Performs any better
Let us know if that helps
Regards
~Akash