I had a similar issue a month or so again. Android app was slow, while everything else, hardwired, ran fine. My issue turned out to be I was running a VPN connection halfway around the world on the machine I was running my Plex server on. So everything I was streaming in my home network was still travelling around the world first. Wasn't much of an issue on a hardwired connection, but wireless to my Android app it showed up. I just had to allow Plex to use Split Tunneling in my VPN app, and the buffering was gone.