If you are playing locally, it doesn’t matter what your internet speed is.
Plex is not Xbmc.
It’s not Vlc either.
As far as transcoding, that entirely depends on the content/codecs involved, and the player’s compatibility with them.
As far as the server transcoding performance, it depends on the hardware and codecs as well.
Anything involving hevc/x265 generally takes too much cpu to transcode in real time.
Gpu transcoding requires a new enough compatible gpu.
For anything 4K/hdr, please read the whole entire thread at [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k
What does your plex server dashboard show when you’re playing a problem video?
You can post a screenshot into a reply.
Or post server logs, from plex web > settings > troubleshooting > download logs > attach the zip to reply
