Currently in Photos in Sierra 10.12, when you drag a 2160p iPhone 4K h.264 video out of Photos, it transcodes it to 2160p h.264. Despite the format being the same, it does this to reduce the file size I guess. For h.264 it's not necessary IMO, but that is the default behaviour, and it can bring even my iMac 4.2 GHz to its knees. Well, not quite, but the way the interface works it almost feels like the machine has hung, because it doesn't tell you it's transcoding. I am assuming this is a software-based encode. To get around this you can export the original (which is a two step process). But I'm curious. What does it do on High Sierra? Does it do hardware encoding for such 2160p transcodes? Or is it still software h.264 encoding? Or is the default now HEVC and it does h.265 encoding instead? h.265 would be even more taxing if they didn't utilize hardware encoding. In the very least I would hope that Apple adds a message indicating Photos is transcoding for export, so people don't think their computers have crashed.