Plex constantly buffers videos it used to play fine

Server Version#: Version 1.42.1.10060
Player Version#: Version 1.112.0.359-0d79a49f

After a fresh Windows 11 install on my client PC, Plex constantly buffers now, and in the dashboard the connection seems to throttle. This never used to happen, everything is hard wired between my server and client with Gigabit connections. Previously the files played fine with no interruptions and faster seek and starts. Playing the files over the network with VLC or MPC yields flawless playback as well, so something in Plex is broken. I have made sure my Network stack is manually selected to gigabit. I have tried running an older version of the Plex Player for Windows. I have tried configuring mpv.conf with:

cache=yes
demuxer-max-bytes=1000M
cache-pause=yes

This hasn’t worked. I have tried updated MPV manually, this hasn’t worked.

I have video quality set to maximum for internet and home streaming. I have Direct Play and Direct Stream enabled. My transcoder is completely disabled.

I have now deleted all previous attempted fixes and accompanying files and am running a clean install of the latest version of Plex Player for Windows as a starting point for any suggestions.

The same buffering issue also occurs in Plex running in Brave Browser. As well as running version 1.71.1.346-f62ce923 of the the Plex HTPC app for Windows.

However Plex on my phone plays the same files flawlessly, although I am running an old version (10.26.0.2578) of the android Plex app because the newer versions break live TV.

I noticed the latest server version advertises: “(Playback) Better caching of codec testing to improve start time of playback sessions (PM-3913)” so I wonder if this has something to do with it?

I’m a Plex Pass subscriber and I can’t say I’m too happy about a completely broken experience. Any ideas or solutions would be greatly appreciated. Have wasted about 6 hours already troubleshooting.

Turns out an advanced setting on my network adaptor was causing speed to be reduced, I disabled Jumbo Frame and Priority & VLAN and proper gigabit speeds returned. Now videos are playing smoothly again with no buffering!