In MacOS Chrome, if “Browser” process (according to Chrome Task Manager) is using 90% CPU, what URL or extension is the culprit?

Sometimes when I open a new webpage in Chrome, my Mac (Big Sur 11.2.1) will suddenly start maxing out one CPU. But sometimes even after I close the tab (and for good measure, other recently-opened tabs too), the CPU stays pegged at 100% for at least a minute or two. This happened today and I opened Chrome’s Task Manager and it said that the “Browser” process was using ~90%, with one of the renderer processes using ~20%. None of the other Chrome processes were >10% CPU. It persisted for at least a minute before I got impatient and quit Chrome, after which the problem finally went away. When I re-open the same tabs, the problem doesn’t recur. It’s intermittent and seemingly random.

When this happens again, how should I diagnose which page or extension is problematic? Related questions:

  • What runs in the “Browser” process as opposed to the various tab and extension processes?
  • Is there any way to tell which URL or extension is chewing up cycles inside that “Browser” process?
  • If a page is running poorly-behaved JavaScript, will closing the tab kill the script? If not, does it eventually get killed after the tab is closed? If yes, what’s the timeout?
  • Is it possible for CPU to be be consumed by tabs that I’ve closed? If yes, in what circumstances could this happen?

Note: this is not a dupe of How to find which tab a particular Chrome process refers to. That question is about basic tab resource troubleshooting. This question is about what to do when basic troubleshooting with Chrome Task Manager isn’t helpful.