How to make KDE use less RAM

* disable akonadi
(config file: ~/.config/akonadi/akonadiserverrc)

line:
StartServer=false

* disable baloo
(config file: ~/.config/baloofilerc)

line:
Indexing-Enabled=false

* uninstall packagekit
(sudo dnf remove PackageKit.x86_64)

* stop / disable packagekit service
(sudo systemctl stop packagekit.service)
(sudo systemctl disable packagekit.service)

* deactivate background services that you don't need
(System Settings > Background Services)