Android app slow initial startup time

I experienced an increase in speed when building the release app instead of the debug version that I normaly run.

I'm not sure why it works, but I guess it has to be because of the way the compiler links the ext libs into the apk. I once saw an interview with Chet Haase who is one of the devs on the android ui platform, who explained how they were trying to show the first activity on the app as quickly as possible to avoid dull splash screen. Maybe that feature is somehow enabled in the release build process.

EDIT: The correct answer is written below by @Embydextrous. It is cause by the dexing of the app in debug mode.

This usually happens with debug builds but can also happen with release builds.

In case of multiple dex files. The primary dex file (classes.dex) is loaded first which then loads other dex files.

It usually does not show in release builds because of proguard which removes unused methods and reduces method count so only a single .dex is made. In debug build proguard is not used so there are multiple dex files.

However in very large apps like AirBnb, Facebook, twitter, etc. there are multiple dex files. And hence app launch delays which can be optimized using dex optimizers.