In modern processors the bottleneck generally is the IO. While it is not the same think (desktop vs phone processors), this author benchmarks OpenSSL in an algo

In modern processors the bottleneck generally is the IO. While it is not the same think (desktop vs phone processors), this author benchmarks OpenSSL in an algorithm similar that Android uses, with and without AES-NI (Intel HW encryption): https://mjanja.ch/2013/11/disabling-aes-ni-on-linux-openssl/ https://mjanja.ch/2013/11/disabling-aes-ni-on-linux-openssl/. Even without AES-NI, this processor is doing more than 100MB/s of encryption. Mobile storage is a lot worse than this (like less than 50MB/s in sustained sequential writes), and you have to remember this would be the worst case (since the majority of writes for common users is small files instead of big files). Just to put in perspective: my Moto X can write around ~20MB/s when transferring files by USB 2.0. And it can still write the same ~20MB/s with encryption on.