Video stuck and resolution is low

I am learning Angular 4: Beginner to Pro, and I usually use the 2x speed to watch the video. Recent month it was not smooth as before, I checked my internet speed and it doesn’t change. The video looks stuck frequently and the resolution is as low as 320p or lower. I was wondering if that may be due to the server of the website or other reasons. Have you guys experienced a similar problem?

Browser: Safari

It seems better on Chrome. Just found there is no quality option in the setting compared with Chrome.