The WiFi on planes is satellite and something like 256 Kbps with extreme latency. It works fine for light web browsing and email, but it's unusable for video streaming. Your options are to pre-load entertainment or watch what the plane has pre-loaded in the seat-back entertainment screens.

This is changing. Gogo has new technology that’s supposed to be up to 100 mb/s, though I’m sure real-world will be lower.