To me LR's inability to run consistently is the single largest issue with the program. I am running on a i7 3.4, not overclocked, with 32GB of ram on a 1.5 year old Asus board. OS on one SSD and LR catalog on a separate SSD. I have now a nvidia card 970GX with 4GB of ram. This should be plenty for LR or any current program that is windows based. You can check or uncheck the "gpu" button in preferences, and it makes no differences. In fact the gpu selection in LR seems to make the processing slower and more prone to crashes. Sad state of affairs for LR, when it's sister program CC Photoshop to me is just the opposite, runs, fast, never crashes, zooms in and out to 100% in microseconds, no hesitations and can operate with a 30" screen at a 100% view.
All other imaging programs I run, CC Photoshop, C1, Premier Pro, have no issues on this machine. However with LR 5.x an now 6.x I always see bog downs, and slow downs. I use a 30" monitor, and you can forget using LR on a full screen as it will really blog down on 100% views, moving around an image etc. If you have an image with a lot of adjustments, then this just gets much worse. I have re-loaded LR, totally, and still it has to me major issues.
I am starting to believe that a lot of other users, just use LR to make a raw conversion and catalog, and don't seem to seem to use all the tools. I am just the opposite, as I now do (attempt to in LR) the vast majority of my work in LR. Take a D810 or Fuji X-T1 file add 3 to 6 adjustment brushes and a ND filter and just watch the performance drop. This issue is not new and has been mentioned before but for some reason Adobe just doesn't seem willing to fix it.
Paul C