the slowdown is with 4k movies - all codec (audio/video) match so no transcoding, plex runs in a vm with 4 typically idle cpu's allocated, and disks are cached

the slowdown is with 4k movies - all codec (audio/video) match so no transcoding, plex runs in a vm with 4 typically idle cpu's allocated, and disks are cached with ssd's on a synology. still the slowest of the players I watch. you can use infuse, and often times it is much faster than the plex client against the same server.