I just went from RC1 to RC4. My scenes with Forest Pack 6 slowed to an absolute crawl in the viewport, even without IR running. When I started IR it was like it wasn't making any progress (continual restarting mentioned above?)
I just went back to RC1 and my viewport framerate jumped from 5fps to over 200! Clearly an issue with Forest Pack and the latest RC.
Max 2016, latest service pack, windows 10, gtx 1080 Ti on an AMD 1950x.
Thanks,
Daniel