Today, my builds suddenly started taking a long time to install & launch on my iPad (using XCode 7). During the "Performing Install Actions" phase, it hangs for about a minute, and then when the launch screen shows on the device, it takes about another minute for the app to launch. After the app launches, it runs normally without any issues.
I've restarted Xcode, cleaned the app, deleted build folders, deleted DerivedData, restarted OSX, restarted iOS, deleted the app from my iPad, and even upgraded iOS on the iPad to 9.0.2.
Edit: This issue originally seemed to be specific to the iPad Mini 3 I was using; it didn't happen on the iPhones I'm testing with. Now, after the weekend, it's happening on an iOS 8.4 iPhone 5s that I'm also developing on.
Edit 2: This was happening sporadically yesterday and it now seems to have cleared itself up, on all my devices. Still no idea what steps to take to fix this if it happens, though.
Edit 3: It continues to happen, but much more rarely.
I've been dealing with this same thing, but connecting the phone to WiFi seems to unblock my installs. Who knows why... but give that a shot if you're still running into this issue.