Sharing video youtube is slow. Any idea ? thanks !

I have no problem of video and audio with zoom but when i share my screen to show youtube videos on the internet, the videos are slow.

I don't have any audio issue with the youtube videos, only the video quality is not good for the people watching it on zoom. I tried everything, searched everywhere, checked my internet connection... I am using a mac book air, OS Sierra 10.12.6

If someone has an idea. Thank you !