While newer versions of Android exist, 5.1 is often considered the "sweet spot" for this specific hardware:
Replaces Dalvik for faster app launching and smoother animations. rk3188+android+51+firmware+better
Most RK3188 devices shipped with Android 4.2.2 or 4.4.4. Upgrading to is often the "sweet spot" for these legacy chips because it provides: While newer versions of Android exist, 5