I have a Kindle Fire HD 8 Tablet. Amazon has clearly optimized the software for it. For example it has three types of memory. Flash for storage, RAM for main memory and (presumably) slower RAM that is used as swap space. Most Android devices run a linux kernel without any swap space configured. But the Fire Tablet swaps (which is why it can run Android 5+ with only 1Gb of RAM I suspect). It has two processors. A slow processor that uses less battery, and a faster one which is switched on when needed.

My point is that Amazon tried really hard to give a good experience with lower end hardware then most Android devices. Encryption likely adds a significant performance cost.

Keep in mind, the iPhone has hardware support for its encrypted memory. It is my understanding that the main processor does not get involved in the actual encryption of the flash storage. Because Apple controls the hardware and the software, they can do this tight integration. Android is software and encryption on Android must be done in software unless and until a hardware vendor integrates bulk encryption into their memory system and provides an appropriate driver for Android to control it. If such a device exists, I haven’t seen it yet...