are you connecting to a plex server at your house? then the speed from your provider doesn't matter, as that only applies to traffic that originates or ends outside your home.
Is the video direct playing or transcoding?
If you're playing from a remote server, then their connection speed also comes into play, and a lot of times, upload is much slower than download. 20Mb/s is not unheard of for upload, and if they're streaming to more than one person, that's easily filled up.
If it's transcoding, then it could be that server isn't powerful enough to process that stream.
Get speed test, not sure about your android box, but there should be an app, or from a web browser if available. That will let you know if you're having network speed issues.
It could be a wifi issue, you may be connected to the wrong band on your androidTV. 2.4 is slower than 5, but has more range. 5 can be faster, but doesn't handle obstacles like walls and furniture as well. So if your router is on the other end of your house, then it's possible that the 2.4 will be faster than 5 in that location.
If the streaming box is buried in a cabinet, or very close to the TV or other electronics, that can also affect the wifi. Try moving the box away from other electronics. If it's behind the TV, sometimes moving a few inches either way can improve performance, depending on what's located inside that particular location on the TV.
If it's an option, try hard wired ethernet.
