Xcode is running really slow

Same issue for me. I had execute these instructions two times before the app would launch without hanging for a minute.

1. Unpair your phone from xcode
2. Launch xcode with your phone not plugged in
3. Plug your phone in
4. In xcode, under devices, unpair your phone
5. Stop xcode
6. Run the following commands

• rm -rf ~/Library/Developer/Xcode/iOS\ DeviceSupport
• rm -rf ~/Library/Developer/Xcode/iOS\ Device\ Logs
• rm -rf ~/Library/Developer/Xcode/DerivedData

7. Start xcode
8. Plug phone in

When you build a project for the first time do it normally (it will take a while like you described)

After the first build Go to Product -> Scheme -> Edit Scheme. Select Build in left side column and uncheck Find implicit dependencies.

This appears to be a well known problem: https://forums.developer.apple.com/thread/62737

"If there's not enough context to decide, the compiler has to go with the conservative choice and record it as cascading."