
If you already know what a GPU is and does, skip this section. However, it seems that there are many combinations of factors that make it not work well, so disabling the new processing feature may help you get back to the speeds you saw in Lightroom 5.

If you want some more technical details, check out this Adobe forum post. For some having this feature enabled works, making many things in the Develop module go significantly faster. It makes Lr CC 2015 work more like Lr 5, sidestepping what was supposed to be a significant enhancement to improve the performance of the sliders and other tools in the Develop module – especially for 4K and 5K high resolution displays. On the Performance tab un-check the “Use Graphics Processor” option.Choose Edit > Preferences (Windows) or Lightroom > Preferences (Mac OS).Getting straight to the point, if Lightroom CC 2015 is so slow it is really not even usable, try disabling the GPU acceleration in the software: Here is something to try if Lightroom CC (version 6) is REALLY slow for you.
