XCode 12.3 running simulator very slow

After a update Big Sur 11.1 my XCode running very slow and with Simulator to debug it takes for ever(1-2 hours) to start-up.

What can i do to remy this?
Did you try reboot the mac ?
Yep did so 3 times now, still no improvement.
Is there any security software installed on your Mac? If so, take a look at the Xcode 11.2 Release Notes:

Known Issues
Third party “endpoint security” software may cause slow simulators, system freezes, or prevent debug processes from running in simulators reliably. This sometimes manifests as debugserver disconnections or simulator applications receiving a SIGKILL signal. (55853555)

Workaround: Uninstall the third party software.

If that doesn't apply to you, please use Feedback Assistant to open a bug report.

It seems like in some cases Xcode 12.3 launch way to many swift compilers and starts swapping heavily. Check for memory pressure in Activity Monitor. If it's red, try to fall back to Xcode 12.2 (helped me).
It seems that deploying wirelessly is causing this. Try to go to Devices an disable “Connect via Network”.
XCode 12.3 running simulator very slow
 
 
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