Our CDN, the Google Global Cache, serves a massive amount of video, and migrating it all to HTTPS is no small feat. Hardware acceleration for AES is widespread, so we were able to encrypt virtually all video serving without adding machines. HTTPS improved quality of experience on most clients: by ensuring content integrity, we virtually eliminated many types of streaming errors. Ads on YouTube have used HTTPS since 2014; we’re also proud to be using HTTP Secure Transport Security (HSTS) on youtube.com to cut down on HTTP to HTTPS redirects.”]
Source: https://security.googleblog.com/2016/08/adding-youtube-and-calendar-to-https.html