Where are the image files stored?
-an external drive
>2X GTX 1080 video card
That card is VERY new, and I have read other messages indicating problems (don't have that card, so didn't save any links)
Exactly which NVidia driver version?
From the Premiere Pro forum... use driver 368.69 or later to fix the nVidia problem with R3D files
Try and turn off the GPU from the Lightroom Preferences.
Go to Lightroom Preferences from the Edit menu
Click on Performance tab
Uncheck the Graphics Processor