Very pixelated online video playback on Macbook Air

[twasnoon]

What can cause the online video playback to become very bad quality and pixelated on a Macbook Air? It happens in Safari, Chrome and all other browsers I tried. The exact same videos from the same website sources look fine on other notebooks using the same wifi connection, so the problem is not with the video source or the wifi connection. I tried to clear the browser cache, reinstalling flash player, changing the display color in system settings, and even reinstalling the op system (High Sierra), but nothing helped. Offline videos look fine on the Macbook Air, only in browsers the video quality becomes unwatchable. I hope somebody here can give me a tip about what can cause this, thank you.