Earlier quoted context omitted.
That's just because Chrome throttles JS execution in background tabs to a crawl.
Thanks. Is there a fix?
Random aside: I just noticed that after you drag the Gangnam tab out to its own window, then you can open other tabs in that same window, and it doesn't put the Gangnam tab in the background anymore. But if you put the tab/window back in the original set of Chrome tabs, then it will.