There is a long-standing bug in lighthouse where out-of-process iframes are counted against the main thread. This causes major degradation in lighthouse scores.

There is a long-standing bug in lighthouse where out-of-process iframes are counted against the main thread. This causes major degradation in lighthouse scores. Even YouTube’s own iframe embed is affected. You could have a perfect-scoring page, put a YouTube embed on it, and the score will be decimated.