Determine which Opera tab uses 100% CPU?

It seems that the answer is available at last! So I have no second thoughts about answering my own question ;)

To the point: development snapshot of Opera 12 (12.00-1359) introduced the opera:cpu tab. Opera 12 hit the virtual shelves on June 14, 2012, so problem is solved. It's not very detailed, but completely sufficient for answering my initial question. Looks like this:

Nowadays, Opera uses the Chrome engine, so just press Shift-Esc to see the CPU usage of each tab.