for those of you who have tried Adobe's suggestions without success, here is a solution to a bunch of problems I experienced with LR CC. 1) very slow previews; 2) no histogram a in library or develop module; 3) unclean app closure requiring a "force quit". Mind you, these are on fully up to date Yosemite operating systems. My solution. 1) uninstall LR CC using the Creative Cloud uninstalled. 2) reboot your system. 3) bring up the Creative Cloud app. 4). Install LR CC. 5) when the installer asks whether you want to delete old preferences or reuse the preferences CHOOSE DELETE OLD PREFERENCES. (This is the crucial step). 6) when you first load LR CC after this, your various preferences will have to be reset, but you will have an u corrupted preference file. I've concluded that many of the issues unrelated to the app opening and closing immediately seem to be related to the preference file getting corrupted during the catalog upgrade. This solved every one of my problems and left me with a clean and fast running LR CC.
i didnt bother to search out the name or location of the preference file, but I'm sure that if you were able to locate it and delete it before running LR CC (and possibly LR 6), it would accomplish the same thing. My approach above is foolproof in the sense that you are sure to delete the correct file.
hope this helps.