By default, it tries to access your data on every website. So this youtube extension is running no matter what domain you're on. And it turns out, that breaks a lot of websites. If you don't want a headache, go into permissions and restrict it to only run on youtube.

It does its job very well, but recently it has slowed down YouTube dramatically. Longer to load videos, pause, etc. It will sometimes take more than 10 seconds just to pause the video I'm watching. Issues are resolved if I remove the extension.