I think I solved it. I Googled the Chromium version from the changelog above "Chromium 147.0.7727.56" and found this Reddit thread:
https://www.reddit.com/r/chrome/comments/1snk7pb/massive_lag_on_startup_since_new_chrome_update/
Several users reported uninstalling and reinstalling the Bitwarden extension as a potential solution, and it worked for me as well. I uninstalled it while still on 131.0.5868.0 and upgraded to 131.0.5877.0. Opera was responding normally this time.
I then reinstalled the Bitwarden extension from the Opera store and it's still OK after restarting Opera again. So I guess problem solved for now.
So if anyone runs into this issue, try uninstalling and reinstalling the Bitwarden extension.
