Background video landing page pixelized the first seconds, 7.0

Hello everyone, 

The video background of my landing page (link Youtube) is pixelized the first 4 seconds then it get focus. 

I have a template 7.0

The video is 20Mo for 20 seconds long. 

I don't understand why the begining is low quality. Do you have a clue or had been faced the same problem ? 

Thank you very much 🙂