I recently came upon this issue after I got my Chromecast Ultra and downloaded some 4K movies. I noticed they were all stuttering and buffering constantly, to the point that it was unwatchable. I tried a bunch of suggestions I found here, but none of them worked. Looking at the Task Manager on the PC running the server, I also noticed the PlexTranscoder was taking up 75% of my CPU power!
On a whim, I tried casting from a browser on that same PC. No buffer or stuttering, beautiful 4K video, and Task Manager shows maybe 2% usage for the Transcoder.
Why does this happen? Is there anything I can do to fix this without sacrificing quality?