I believe this is new to Mavericks: Here’s an easy way to identify the windows in Safari that are currently using resources.

Open Activity Monitor, and then hover over a Safari Web Content item in the window. That will display the open URL(s) belonging to that instance. This makes it easy to identify the window in Safari that is using up your CPU.

Lex adds: This works as described, and is great.

Source: Mac OSX Hints