RAW from d800
loading full previev on LR5 ~~5s
loading full preview on LR6 ~~20s
tested on many photos,
this is 10x more speed up?
on new imported photos from camera it working excelent FAST, but on old catalog is VERY SLOW, it will be fixed?
Have you got the GPU performance option enabled (Preferences>Performance tab)? If so, try unchecking it and testing with the older pictures again. What are the loading times then?