I am getting very similar slow performance using CC2015 and ACR 9.1.1 on a i7 win 8.1 with 8gb of ram on a QHD 3200x1800 screen. I've disabled GPU but it's taking 3-5 seconds to apply any adjustments like straightening or adjusting white balance. Moving from one raw file to the next it takes 7seconds for the image to render sharply. With GPU enabled this was over 11 seconds, so it's a definite improvement - but what else can I do?
I bought this laptop to be my powerful yet portable field laptop for processing while out and about and at the moment it's slower than my 2010 i3 with 4gb of ram.
I'm getting very slow screen response editing in Camera Raw.
It's back to the speed I had years ago on files of the same size.
Can't really use it for work at this speed.
OSX El Capitan
Late 2013 iMac w
32 GB of RAM
Nvidia GTX 780M video card with 4BG of video memory
Lots of space on my 1TB SSD drive.
I resolved this problem to a great extent by turning off "Use Graphics Processor" in Camera RAW prefs.
Performance is much better.
There appears to be an incompatibility between either Camera RAW and the video card or wth OSX El Capitan.
I also purged my Camera RAW cache, for whatever that's worth.