Why android OS performance is not too efficient compare to iOS?

The problem is specialization!

Apple's iOS is optimized for a small set of specialized Apple processors, which are ARM-based but because of e.g. custom extensions are able to do a lot more than the specification. Each app is compiled for every set of hardware (like one binary per CPU+GPU combination).

Android needs to work on many different CPUs (Qualcomm, Exynos, MediaTek and all there different models) and thus trades performance for support and availability on all those devices.

With the latest addition of ART, Android tries to gain back a bit of the sacrificed performance by recompiling the binary (from the Dalvik bytecode) with vendor specific compilers that allow them to specialize the binaries again.

However, apps that rely on libraries compiled with the NDK (Native Devlopment Kit) see no improvement when using functions of them, because the binary files are not recompiled.