Xcode 12.3 is not responding a few seconds after opening it
This is a recently reported bug with Xcode 12.3. I am not aware of any response from Apple yet, but you might be able to fix this issue by turning off the over-the-air test device builds.

Top Menu: Window &gt; Devices &amp; Simulators
Uncheck &quot;Connect via Network&quot; for each physical device

I recommend filing bugs with Apple and watching the forum thread about this.
You also revert to Xcode 12.2.