With the latest versions of Safari, your loaded Web pages are collected in groups of separate running processes. This helps keeps other Web pages stable if one goes awry. Here is a quick way to identify which tab/URL is causing the problem.
Load up Activity Monitor and search for “Safari Web Content”. Sort the item by CPU usage or any other method for locating the web page that is giving you problems.
Once you have located the problematic process, simply hover your mouse over it and you will see a tooltip box containing the URL of the page. Unfortunately you cannot click the links in Activity Monitor to bring the corresponding window to the front, but what you can do is use the link information to identify the potential Safari tab(s) that contain the problem. All you need to do is to review your windows, and see which contains tabs for the links in the tooltip. Once done, you can try to manage it without quitting and relaunching Safari, or more intrusive troubleshooting steps.
While this tip is useful for Safari, it should work for other processes that use WebKit, including “Content” processes for the App Store, though these may be more limited in scope (as in, the URLs of the App Store only) and not as useful for troubleshooting.
