Trying to resolve buffering, but no VERBOSE speed data in log

Hello
I'm trying to work out why my Plex is buffering. The media are on an iMac, I'm streaming to an xbox over wifi.
I found a page that told me to turn on Verbose reporting or something and look for data about speed in the log.
The thing is, there is no data about speed in the log.
So what do I do??
Thanks
Nik