[Michael Van metre]

I have the same computer, late 2015 iMac 5K Retina, 32 GB Ram, AMD Radeon R9 M395X GPU.

After extensive research and trouble shooting my conclusion is the Radeon R9 is not compatible with Premiere Pro CC. Adobe has confirmed that they have not tested the card and will not give a time frame as to when. Very disappointing to purchase a “high end” iMac to work in 4K only to have it not work decently at all. I’m looking into swapping out the GPU for one that is compatible with CC and will still run on the iMac, but this is doubtful.

I have been in contact with both Apple and AMD in addition to Adobe. Currently there is no work around. Apple needs to stop selling this configuration as the end all NLE work station when it is not compatible with Premiere, and Adobe needs to get busy and test this GPU and make it work. This has been a known issue for well over a year and a half. Solutions are:
1- wait it out
2-go with a PC
3-try a swap out of the GPU-doubtful
4-edit with FCP X