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How could Google have squeezed more performance and more battery life out of a simple software update, you ask? Well, it comes down to ART, AOT compilation, and Project Volta.

ART, or the Android Runtime, has actually been available since Android 4.4 as an optional developer feature that could be enabled if you so choose. However with Android 5.0, ART is set as the default, effectively replacing the old 'Davlik' runtime that has been around for years. This change has many performance implications.

Davlik used a just-in-time (JIT) compiler to compile an application’s Java bytecode into native hardware instructions when said code was executed (such as when an app was opened). This form of compiler was necessary in older Android editions due to the hardware limitations of the devices it was running on, specifically in the storage and RAM departments.

ART shifts to an ahead-of-time (AOT) compiler, which compiles the entire application’s bytecode into native code only once, rather than each time the application is opened. As these native instructions typically use more space, apps themselves have a larger footprint on Android 5.0 and devices running ART; something that has only really been possible since RAM and NAND capacities of Android smartphones have increased.

By compiling applications into native code ahead of time – typically when the app is opened for the first time – a lot of system overhead is removed for any subsequent launch and use of the same app. Although this does mean the first launch of an app will be slower under ART than Davlik, after the first launch, performance should be improved.

Google also claims that by compiling the entire application at once and only once, it can achieve greater optimization of the code, which again should improve performance. As overhead is removed at the same time, battery life should be improved thanks to fewer CPU cycles going to compilation each time an app is opened and used.
ART also features an improved garbage collection (GC) mechanism, which works behind the scenes to allocate and reallocate memory. Previously, the GC system would have to pause code execution to clean up memory use, which would cause what Google describes as “jank”, or general stutter while using an app. With ART, the GC system has been improved to reduce pause times, in turn reducing jank. Combined with better memory allocation systems, and again performance increases.