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.