Whilst Lightroom 6 has added a few minor extras compared to Lightroom 5 it was disappointing to find that performance is still pretty poor, even with a supported video card.
For users that have been experiencing heavily degraded performance compared to Lightroom 5.7 you may want to check your video card, it looks like Adobe and ATI are not as in sync as Adobe and nVidia, you can read more about the problem on the Adobe site: https://helpx.adobe.com/lightroom/kb/lightroom-amd-graphics-cards.html