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
To be fair to Adobe its likely that trying to get the most out of a range of video cards running with a variety of drivers is not an easy combination to manage and keep working.
Lightroom speed for editing has sometimes been fast enough for commercial use but each point release brings its own slowdowns and speedups and whilst some aspects of Lightroom 6 are better (slider performance in the develop module) I again find that Lightroom is too slow to edit large sequences of images for clients.
Adobe Bridge with a prebuilt cache and 1:1 proxies is still much faster than Lightroom for editing serious amounts of images – which is a shame as both should be able to display prebuit proxies at pretty much the same speed (and Bridge doesn’t even pretend to use GPU acceleration), I think someone fresh needs to look at the relevant Lightroom code and see if they can optimise it.