I had a 27 inch iMac late 2012, used to run LR 5 all fine, then LR CC and all fine too, then upgraded to iMac 5k and now whenever I run LR CC the whole computer slows down. Just moving from one image to another can take up to 3-4 seconds, then if I export from LR I cannot use anything else in LR, moreover if I try to use any other app on computer it takes minutes to mover from app to app, and even when there are only one or two open. Before I could have several things running no issues at all.
If i close LR everything works fine again.
I went into usage monitor and it shows that LR is using up to 9Gb of RAM at times... I only have 8Gb so it is using hard drive to support !!!
PS only uses about 500Mb maximum and nothing else gets close.
Apple say not there problem which it probably isn't
I tried trouble shooting on CC and adjusted GPU setting to ignore and no change, I checked and the graphics card is compatible with CC, it is a brand new 5K so after all it should do. Even checked the card capability with Apple against CC spec and they confirmed that it is more powerful than required.
So what do I do ???
Tell us what the graphics card is. Tell us what your CPU is.
Have you checked the box in Lightroom under Preferences/Performance Tab that says "Use Graphics Processor"?
Are you doing a lot of brushing or spot healing on individual photos?