Thanks to those people that provided potential solutions to my first problem (high CPU usage).
My second problem is with network access.
Initially I had an issue with server mapping - it kept telling me that the client was on a different subnet to the server. Thankfully I have fixed that problem.
What I haven't been able to fix is access from outside the network.
I have tried the help offered by Plex (by clicking the "Learn more" link shown in the screen snap below) but I am unclear of what to do.
I did try specifying a public port (I think this is what helped resolve the subnet issue, but can't be sure).
I also enabled UPnP on my router (maybe THIS is what resolved the subnet issue) but Plex still won't connect to the web at large.
This screen snap shows my network settings in Plex:
Clearly, I have no idea what I'm doing. Reading Plex's help advice hasn't helped me... maybe someone else can explain where I'm going wrong.
Thanks.