Disable Search Indexing in options. It uses a buggy piece of garbage called Akonadi in the background and was causing one core of my CPU to stay at 100% for no reason.
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.
Akonadi is a pig. Nearly 20 processes, each one using 20-150MB resident set (20-40MB unique set), multiplied by the number of users logged in. And then there's the other stuff it keeps resident, like mysqld.
That might be okay if I was getting something important from it, but I'm not. It provides zero value to me. It's just wasting RAM that I would rather use for other things.
Unfortunately, it's part of the Plasma dependency chain on my distro, so removing it would be problematic. When I find the time, I may build a custom metapackage to allow me to get rid of it without taking most of KDE with it.
Akonadi also hogs a lot of memory for services i never use (calendar and centralised mail service, not sure if thunderbirs uses them). It's not a problem for my desktop pc so i don't tinker with it, on my 8 year old laptop on the other hand I'm going to have to switch to something lighter in the future (lxqt or xfce).
I'd love it if the KDE devs made Baloo and Akonadi optional. Their insistence on including them reminds me of Micro$oft's insistence on bundling Internet Explorer and integrating it into the OS shell in Windows 98.