I think I asked this question before. Below is the answer I got.
Hi,
the problem is iOS itself!
See here:
http://krpano.com/ios/bugs/ios8-webgl-video-performance/
and here:
https://bugs.webkit.org/show_bug.cgi?id=129626
According the bugreport above Apple has already fixed that, but there is no iOS version available with that fix yet. Maybe this will come with iOS 10 or iOS 11...
Best regards,
Klaus