OpenWRT running on a dual-core (MT7981 2xA53) handles Plex Server smoothly.

Based on the information provided in this post, I installed Plex Media Server on my RAX3000M Router, which is equipped with only a dual-core processor, 1GB of RAM, and a USB 3.0 HDD attached.

https://forum.openwrt.org/t/plex-media-server-for-openwrt/179676

No transcoding is required; the content is played in its original quality. Take note of the CPU usage while the client is playing a movie at its original quality. Additionally, when transferring files from my Mac to the Router’s attached USB 3.0 HDD, the speed reaches up to 80MB/s.

easy steps:

copy the code to /etc/init.d/plexmediaserver

opkg update && opkg install unzip curl squashfs-tools-mksquashfs
service plexmediaserver start

and if you got this error:

mv: can't rename '/overlay/.plex/Library/Application/plexmediaserver.*': No such file or directory
Plex Media Server XZ-compresssed SquashFS (.sqfs) archive generated!. Archive is located at: /overlay/.plex/Library/Application/plexmediaserver.sqfs
setting up / checking / loading UCI config for plexmediaserver
/sbin/uci: Entry not found
/sbin/uci: Entry not found
/etc/rc.common: line 247: can't create /overlay/.plex/Library/Application Support/Plex Media Server/plexmediaserver.pid: nonexistent directory

The solution is to manually create the directory.

mkdir /overlay/.plex/Library/Application\ Support
mkdir /overlay/.plex/Library/Application\ Support/Plex\ Media\ Server
service plexmediaserver start

enjoy!