Hmm. Maybe it's all a coincidence. When one of my CPU cores was stuck at 100%, I opened htop and configured it to show kernel threads too. I spotted MariaDB running in the background. I thought "I don't remember installing MariaDB". Went to uninstall it with pacman, which said it's a dependency of Akonadi. After googling, I turned off Search Indexing and CPU usage dropped to zero. I'll keep an eye on it to see if the problem comes back.