The representative of Google's John Mueller acknowledged that embedded YouTube videos can slow page load time.
However, he noted that the search engine does not distinguish between inline elements of their own services and competitors sites. He also added that the YouTube players must not reduce the loading speed too much, as they are well optimized.
The question to Mueller was:
On my page many YouTube videos. Whether it can reduce its loading speed on mobile devices?
He said the following:
Maybe. We try not to make any exceptions for own services. Accordingly, if you are using YouTube videos, AdSense ads or any other built-in elements on pages, and the speed of download is reduced, we will inform you. It doesn't really matter whether it was Google or you publish a video on your website. We will treat them equally. Players YouTube are usually pretty well optimized. So adding them to the page is unlikely to significantly reduce the speed.
Listen to Muller's response, starting at 45:13 of the video below:
