Even if the port is open, Plex can sometimes use the relay server. You might want to explicitly disable that.
Also, you can check if the video is being transcoded on-the-fly by going to quality settings and seeing if it's showing up as transcoded or directplay. Sometimes you might have to force the directplay on your client device (Playback might fail if your device doesn't support the specific format).
As someone who tried to set up Intercontinental sharing with Plex, I found out two things that could be an issue. One would be the relay option. That's somewhere within your server. You might want to disable that. That should then allow you to directly connect to your home server.
And the second one was that the client device that I was using didn't really support the format that I want to look at and that ended up just making it so that the video was transcoded that is going to degrade your performance. If the second thing is happening, you might want to check your quality settings to see if you're using directplay. If not, Plex might transcode on the fly to a lower quality to mitigate buffering.
Have you tried being filthy rich?
Good thought, that way you can make your phone look massive.
Hold on, I think I got it the other way
You can set it application specific in the window decorations settings. Not sure about a global toggle though.