Mac: With Activity Monitor, you can quickly see which of your apps is hogging system resources and shut them down as needed. But Mac OS X Hints points out that in Mavericks you can get even more granular and see which tab in an app like Safari is slowing down your machine.
All you need to do is open up Activity Monitor and look for "Safari Web Content". Hover over that item with your mouse, and it will display the URL in the that tab. It's an easy way to identity which Safari tab is hogging up your CPU.
See open URLs per Safari Web Content instance in Activity Monitor [Mac OS X Hints]