In the event that the site doesn't need to load the entire video into RAM prior to starting playback, you could try turning down the video definition from the default 1080 to 720 by clicking "Account" > "Billing & Settings" then scrolling down to the bottom of the page and choosing the new setting. When I did that a few years ago (to prevent sticking and freezes), the change had no significant degradation in video quality but I don't remember if it helped the loading time.