Late 2012 iMac 2.9 GHz Intel Core i5, 32Gb RAM
Mac OSX 10.11.6
What is the exact version NUMBER of Lightroom? Do not tell me "up-to-date" or "latest version".
Release 2015.7 Camera Raw 9.7
I am working with Nikon NEF (raw) files.
Try turning off the GPU acceleration
Go to PReferences, click on the Performance Tab, uncheck the box
Thanks - will give that a go!