@j.p.s: I've yet to see a Kindle that's actually sluggish at its main use case: reading
. On the other hand, look at a Kobo trying to struggle through an ePub, that's fun (not).
(Yeah, yeah, apple, oranges: the format itself is freakishly terrible to optimize for, especially when you're beholden to Adobe's basically unmaintained rendering engine. Things are generally okay-ish when you drop the Adobe bit of the equation by switching to kePubs. Note that, yes, it did get a wee bit better over the years. I still highly recommend not trying to use a Kobo right next to a Kindle
).
Speaking of Kobo, they *did* move to i.MX6SLL boards (from NTX, as usual for Kobo), a few months ago (basically since the Clara). They did not go with an i.MX7D for their foray intay Oasis-territory, the Forma.
TL;DR: Nothing's ever perfect. You just have to choose what you're willing to compromise on.