On iOS system libraries (e.g. UIKit, CoreFoundation) are packed into a file called the dyld shared cache. To get symbols for them, you need to download this file, split it apart, and parse the symbol tables embedded inside. These symbols change with every iOS release, so it needs to be done every time you update. When you see this message in Xcode, it's downloading the shared cache file off your device (which is several gigabytes) and splitting it. This does not get cleaned up unless you do so explicitly (which is why a lot of "dev cleaner" apps will prune it for you). You can also see the symbols you have cached on your computer from System Settings.