You're actually trying to watch videos while simultaneously playing Tetris on a 5.5" screen with an incredibly slow and laggy device?
I'll give you the benefit of the doubt and assume that was just a poorly-executed joke or troll attempt, because that's the most moronic example of split-screen multi-tasking imaginable.
I love my iPhone 6, and I think the 6+ I have on the way will be the best phone out, but I had a Note 3 that I used as a secondary device and could split window such apps with zero lag.