Most Android phone have external SD card these days. if user have installed any apps on external SD Card then they won't execute without inserting sd card on device. User should take backup of any data like contacts, messages and call log on external SD card. They have capacities of around 8GB, 16GB, 32GB, 64GB, 128 GB and performance speed up to 30MB per second in reading and lower in writing.
Latest device with high speed of reading or writing, could be able to take advantages over this new card speeds. Operating system version also affects SD card reading and writing speed. UHS-I (Ultra High Speed) external SD card have new technology and supports high speed reading and writing. Manufacturers sometimes ship the device with under optimized drivers, and third party developers creates an improved driver that is better optimized than the one that ships with the device. Installing custom driver is only possible if device is rooted or if user is flashing custom ROM. Most of the mobile phones write speeds are lower than supported by SD card.
When Android starts, the dalvik virtual machine looks through all android applications and frameworks, and builds a tree of dependencies. It uses this dependency tree to optimize the bytecode for every application and stores it in the dalvik cache. The applications are then run using the optimized bytecode. This is why when user installs a new ROM, sometimes it takes a very long time to boot for the first time. When an application changes, it's reoptimized and the cache is updated.
Defragment flash media have no advantage, because access to any flash cell takes the same amount of time as access to any other flash cell. The reason defragmenting is beneficial to mechanical hard drives is because it makes related data blocks contiguous, so that they can all be read in order as the disk is spinning and minimizing the amount of movement needed from the drive heads. Flash media has none of these moving parts, and therefore does not suffer from the same problems as mechanical drives. Defragmenting flash media could potentially be harmful, because it will slow down the memory faster while not providing any benefit. Defragmenting storage IC memory is not recommended as it may actually break the IC. Unix systems don't need to defrag since the files are already arranged in such a way that it is faster to access them.