If rtkit is spamming your logs with garbage like, "Supervising 12 threads of 7 processes of 1 users.", its log level is probably set to "debuginfo".
Set the max log level to 6, which is "info", and will be much less noisy. On most systemd distros, you can copy the default daemon configuration from "/lib/systemd/system/", to "/etc/systemd/system/", and add "LogLevelMax=6" under the "Service" section. You may need to run systemctl daemon-reload, and possibly restart the service, afterward.
If you don't know how to read logs, and don't understand why log noise is an issue, I strongly advise you to learn about system logs, and how to read them. They can tell you a lot, especially when something goes wrong, and can even hint on what you need to do to fix issues. Having them full of useless debug messages from services that are healthy can make that far more difficult than it needs to be.
For the record, this isn't even just a Linux thing. Every operating system, including Windows, MacOS, and even awful, restricted OSs, like Android, have some kind of logging system, and understanding how to take advantage of it can save you a lot of headaches.
