Plex runs fine on an old Roku I have even with an airport express getting the signal to it. The trick is to make sure all your computers and media players are hooked up wired even through 10/100 switches, even if those switches are wired to an airport base station to get the data from one part of the house to another they will work better that way. When too many antennas are trying to connect to the limited antennas of the airport base stations, things tend to slow down more. I can even have the Roku and Plex streaming at the same time along with the rest of my devices this way, when I did all this through wireless only things became laggy.
Yeah at the moment my setup at home is all wireless... but, the good thing is, my condo was already setup with wired ethernet in the walls, however those lines all lead to my phone company's lines. But, since I don't have or need a landline phone, I can just make the source have one output and split the lines so that I can have wired access to my router from all the other rooms.
Only problem is; I don't know how to cut and install ethernet jacks! I'm gonna need to find someone to do it for me or pay very close attention to a plain as day youtube tutorial. I'd much prefer having everything wired.