hello,also if thread is old I want to add my experience for people searching for same problem.
I got the same problem and after reading stupid solutions like "open the opera task manager to see what process is high cpu blabla" I solved the issue.
After some time opera,don't know why,do not like big history and local storage file
what to do ???
go in your profile folder (go on opera://about to find yours if you are not expert) and find the files History,Local State and Favicons.
Found them ? ok.
Now close or kill opera process.
Rename those files to whataver (example history.blabla)
Launch opera.
Problem Solved.
I hope it will work for you too,if not,need to check any extension or check also if you hdd/ssd got any error on sectors..some extensions (like one tab or tab suspender for example) write a lot on disk and if you got any damaged sector it will cause high cpu.
Hope it helps
P.S.
this problem poops out in latest updates,before opera gx (2 years ago) never has got this stupid issue.