Title. I just received the iMac 5k today and somehow I got an error when I put 5k resolution on youtube, I haven't updated the OS yet, thought I would make a thread first to re-solve the issue. It plays fine at 4k.
lol the quality is already destroyed when it's uploaded to youtube my friend. Artifacts and compression galore! Most youtube video downloaders don't do anything to further degrade or compress the video, that takes more effort and processing after all. I use an app called 4kvideodownloader, copy your video url into there and it should be easy peasy after that.