Thanks for your reply Jim. My first reply to that would not go due to "an unexpected error" so I am trying again. I am not skilled with computer trouble shooting. I have a 2 year old iMac and am using Lightroom 6. I talked to someone at Apple Support re the graphics card and he was sure that the graphics card would have been updated along with the OS if such an update were available.
I was not able to find anything in the Lightroom preferences re GPU acceleration. I did find one thing that might be relevent. See screenshot below.