Hello @meeffe , this is probably happening with a specific site you have open. You can see which site is causing you trouble by selecting 'View -> Show Developer Menu' in the menu and then 'Developer -> Task Manager'. After waiting a few seconds it should tell you which tab is causing the CPU use.
In particular, it can be due to cryptocurrency mining on a site, which is unfortunately a growing problem. If you think it's due to this, Opera's ad-blocking can help with that in some cases. Enable it in Preferences -> "Block Ads".