Which tab is "Google Chrome Helper" running on?

Chrome's built-in Task Manager (accessible from Menu > More tools > Task manager) will show you a per-tab resource utilization.

If case the culprit is extension code or a plug-in running in a normal tab, you won't be able to identify it with only that; extension entries in the Task Manager are only for background pages of extensions.

You may be able to investigate further with Dev Tools profiler.