Your probably have the GPU acceleration turned on by default.
It is not ready for prime time with many of the AMD video cards.
Goto the Preferences dialog, Performance tab, and turn off the GPU acceleration.
Also, see the other really big threads about the video cards and OpenGL stuff.
I also have a fully maxed out iMac Retina, exactly the same spec apart from a 512GB SSD and similar speed issues when rendering :-( I'm also connected to a 4TB external drive where I store my (CR2) raw files via Thunderbolt and using LR v5.7.
I also have another question which I've asked 3 times without a single reply and you might find the same thing. When I open a raw file from the hard drive and apply an Action in PS CS6 (apply sharpening, reduce file size, apply sharpening again and reduce file size to 1600px) it normally takes less than second, sometimes as fast as you can blink your eye. Import the same file into LR and then edit in Photoshop by right clicking and the same Action takes 15 - 20 seconds which is a real pain. I thought it might be due to adjustments in LR but does the same thing without any adjustments.
The machine cost over GBP3,000 so what do you do when you cant get answers? Is there any other way to get answers from Adobe? Anywhere / anything else to try? .................................
Err, yes I have, deleted the first from LR discussions, deleted another in PS discussions which I reworded https://forums.adobe.com/thread/1937689 and I dont appreciate being called a liar.
Thanks a bunch, most helpful, NOT.
An apology would have been nice and some help would have been even better but I guess thats just not your style.
Doesnt say much for Adobe Communites though and if it was a genuine attempt to wind me up, congratulations, you succeeded ...
You turned up spoiling for a fight.
Attitudes don't fly here.
The only person who has an attitude problem is yourself making unfounded accusations and if you take the trouble to look again at my original post you wiil see that I was NOT spoiling for a fight at all, I was simply looking for help. And I've seen the same arrogant attitude in some of your other posts.
You got it wrong and you dont want to admit it, end of :-)