I'm presenting the same issue with the official release of Xcode 15 on macOS Sonoma (I did a clean install and same result) I hope this can be fixed soon.
I'm not running anything in the simulator and still using a high amount of CPU (about 40% on a M1 Max), in my laptop this means that my battery will drain quite fast and high temperature.
After closing Xcode and Simulator the CPU idling goes back to about 96%.
Here is a screenshot of the processes, this was taken with the simulator idling and Xcode doing nothing: