More specifically, you need to map a drive on the PC to your Unraid server, install Plex server on the faster PC, and then configure the Plex server to use the mapped drive. You will no longer need Plex server on the Unraid server you are just using the standard share features of Unraid.
To map a drive:
1) Go to Start -> Computer -> Map Network Drive
2) Select a drive letter, e.g. M:
3) Browse to your Unraid shared network drive(s), e.g. \\tower\media
4) Check the box to reconnect at login
For the Plex server:
1) Do a standard default Plex server install
2) Under My Library add sections for your media subsets,
e.g. add one named Movies, browse to shared drive to M:\Movies
add one named TV Shows, browse shared drive to M:\TV Shows