@leocg Thanks for your reply. You may be right that I was not more explicit in requesting help. Opera is my main browser and the alternatives are not very attractive, but if it slows down my computer and there is nothing to be done, it is difficult to keep it.
Go to opera://extensions and disable all extensions (especially ad-blockers, VPNs, or dark-mode tools). Re-enable them one by one to identify the culprit.