Ok. I'm glad you ran the test, as that is the clue to your buffering issue.
As to why....
These are all possibilities.
1. Distance from the router. E.g. In another room. I found the wifi signal dropped significantly in my house so use those wifi extenders to give me a good signal. Else everything buffered like mad. I then connected my Droidbox to the extender by Ethernet to avoid any further signal drop out.
2. Congested wifi channels, conflicting with other devices, even a neighbours. You can change channels on the router. There's a wifi analyser tool which used to be packaged with Droidboxes if not there maybe an app on the Droidbox market or Google play
3. If you are any distance from the router, ensure you are using 2ghz over 5ghz wifi connection (or whatever the measure is). 5ghz is stronger over shorter distances.
Good luck.
To hopefully prove the box is ok, try placing it as close to the router as you can and rerun the speedtest and view a movie.