It has nothing to do with the webpage you're on - or even the browser. It means the processor is doing something else and can't do what you just asked it to do. You'd see it on a bare desktop.
Usually, that happens during a software update, or some other "behind the scenes" thing that Windows is doing. A pain but harmless. Just let it run until it's done. If you try to reboot, and you see a blue screen that says "Don't turn off your computer" you know it's a Windows update.
But it can also mean you have an infection and your machine has been co-opted to do nasty stuff. First, close your wi-fi connection. That gets you free of the internet and anything going in or out without your knowledge. Then, run both an anti-virus (like Avast!) and a malware (like Malwarebytes) scan. After they find and remove anything nasty, reboot.
I powered up my travel laptop yesterday to let it catch up on any Windows updates it may have missed since I used it last, and it showed that little blue circle running for almost an hour. So there was a lot going on. And it made me reboot three times in the process.
