I Also have this same issue with safari and while using Facebook app.
videos will stop after playing for 10-20 seconds. They act like they are buffering. If I pull up the same video on the same network from my original iPad the video does not stop. This only happens when on wifi. If I shut it off they load fine. How would a video load fine on cell service but not wifi? I know the network wifi is fast.
Setup:
iphone 6 plus
no apps running in background
shut off phone today and restarted still having issue
on very fast wifi
cache cleared from safari in settings
when I try same video in chrome browser No issues so this tells me it's something with safari. I'm guessing when I have issues with video in Facebook the Facebook app must use some of the safari settings which is where my issue might Be.
MAkes ales zero sense that my original iPad doesn't have problem. Please help.