Xcode 12.3 keeps freezing
I found an answer by Darr on the question "Xcode freezes after upgrade to 12.3" on Apple's developer forums that worked for me:
Unpairing my device connected via network and using only lightning cable seems to have resolved the issue for me.
Xcode > Window > Devices and Simulators > device > Connect via network
After going mad, I fixed this by removing all devices from Devices and Simulators (I had connect via network activated).
For now, it's working fine.
Power off your Apple Watch
Xcode may hang if the host Mac is connected to an iOS device which has one or more companion watches. The hangs occur when the watches aren't fully prepared for development, or if they are experiencing poor wireless connectivity. (72490921) (FB8945320)
Workaround: Power off the watches, or unpair them from the iPhone.
Xcode 12.4 Release Notes
You can force Xcode to freshly prepare a watch for development using these steps:
Power-off and disconnect all iOS device from the Mac.
Reboot the Mac.
Launch Xcode, close all projects, and open the Devices and Simulators window.
Power-on and unlock an iOS device and its companion watchOS devices by entering the passcode as required.
Plug the iOS device into the Mac using USB cable.
Check each iOS device and each watch for any pairing request and approve the requests.
Monitor the status of device preparation in the Devices and Simulators window. If Xcode presents device-preparation errors, follow the guidance in the error message to resolve the error.
Once Xcode finishes preparing the iOS device and its companion watches for development, repeat steps 5 through 8 for each remaining iOS device and its companion watches until you verify that Xcode has prepared all devices for development. You may now open your Xcode projects and resume development.