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.
It's funny because Google has this guy, I think he's called Alex Russel, running around and telling the world which websites are too slow and how to improve web performance.
"Google+ felt slow and bloated performance-wise, despite the design being simple" is what he means.
And yes I agree. Due to the sliding/fading animations and other nonsense, it felt slow and sluggish. By today's standards it would probably feel fast. But for the time, it was the slowest thing Google had created.