I recently noticed my CPU usage was high, and it was due to my open Vivaldi browser. I wasn't sure what tab was causing the issue, so I searched to see if Vivaldi had any tools for reporting this.

It turns out that Shift-Esc will open a task manager, and you can sort on any of:

• Task (a string representing high level things like the browser as a whole, GPU process, worker tabs, and more)
• Memory footprint
• CPU (this was what I was interested in!)
• Network usage
• Process ID

You can select any task to end its process.

I was able to quickly track down the issue to a background worker running for a PWA window I'd closed earlier, and ended the process.