The top question you should ask is what browser you use. If you use Chrome, and the parent uses Firefox, that's why. (Not because Firefox is slow, but because Google uses a lot of Chrome-specific hacks.)
When I watch a video from the main youtube page, then hit the back button, the video keeps playing for several seconds while a javascript progress bar at the top of the screen loads to take me to the main page.
I hate it so. much. Usually I'm hitting the back button precisely because the video became annoying to me, or something else is happening where I don't want to hear it any more, and having to wait several seconds while the video still plays is awful.
