I think there may be different factors at play. First of all, the availability of hardware instructions for encryption primitives. Secondly, some flash controll

I think there may be different factors at play. First of all, the availability of hardware instructions for encryption primitives. Secondly, some flash controllers compress data before it is stored in NAND cells, to reduce write amplification and to improve read/write throughput. Since encrypted data is high-entropy, it does not compress well. Performance takes quite a dive on such controllers. That said, I used FDE on my Moto X 2014 with Lollipop and it worked without any problems or noticeable slowdowns. But typically Kindle has far more low-end hardware.