Ask HN: Why is the Khanacademy website so power-hungry?

I like to brush up on some topics using Khanacademy, because it is well structured and gives quizzes with feedback. One thing I've noticed, however, is that my laptop gets very hot, the fan spins up, scrolling gets laggy, and Javascript elements (quizzes, etc) have enormous latency.

What are they doing in there that causes it to consume so much computing power? I feel bad for anyone who uses the site and isn't fortunate enough to have a powerful computer like I do, because if it's annoying to me it must ruin the experience for them.