Slow app launch time after updating to iOS 14 and Xcode 12

I have the same problem, but solved this by doing the following things:

For simulators running iOS 14, delete folders inside ~/Library/Developer/Xcode/iOS DeviceSupport/ (folder's name started with "14").

For devices running iOS 14, select Xcode window tab, click Devices and Simulators, find your device, right-click to unpair the device

After doing the work, re-run your app, everything should be ok.

References:

• Apple forum post: Xcode 11 hangs while attaching debugger to ios process
• StackOverflow answer from: Can I delete data from the iOS DeviceSupport directory?

I also had this problem, here on Xcode Version 12.1 (12A7403).

The proposed solution to "Unpair" the device in Xcode "Devices and Simulators" didn't work for me (and lot's of people here). Xcode simply keeps crashing when trying that. Hard-reset of the device didn't solve it either.

What did the trick for me: Delete the iOS 14.1 folder inside ~/Library/Developer/Xcode/iOS DeviceSupport. Then simply reopen Xcode.

It takes a while for Xcode to "copy cache files" from the iPhone but once that's finally done, normal debug build and run behaviour is back, without the long delay.

Delete ~/Library/Developer/Xcode/iOS DeviceSupport/ and unpair/pair device doesn't work for me.

Click Edit Scheme of the project target, select Run phase row, and uncheck Debug executable below the Info tab, this solved my problem.