Runtime Broker is a Windows process in Task Manager that helps manage permissions on your PC for apps from Microsoft Store. It should only use a few megabytes of memory, but in some cases, a faulty app might cause Runtime Broker to use up to a gigabyte of RAM or more.
If your RAM use is high and your PC is running slowly, an app may be the cause of the problem. Press Ctrl+Shift+Esc to open Task Manager and then, on the Processes tab, check to see how much memory Runtime Broker is using. If its using more than 15% of your memory, you probably have an issue with an app on your PC. To stop Runtime Broker from using so much memory, select Runtime Broker in the list, select End task to close Runtime Broker, and then restart your computer.