Thanks. Reading the link you provided lead me to a less destructive (compared to the other thread) approach which solved the issue:
mv "~/.local/share/akonadi/db_data" "~/.local/share/akonadi/db_data.old"
akonadictl start # This would take some time, so be patient
Now I can easily use Kalendar, I haven’t rebooted the machine to see if the lag for non-KDE software is gone or not. I’ll try to remember to report back.