I mostly read on an old iPad mini. The screen is good enough for me that I don't feel like I'm missing out by reading on a non E-ink screen.
How did you manage to get it to not bog down in speed? Every tablet I've ever had has become barely functional by year 3, and that includes the iPad Mini.
But yes, even the first gen iPad Minis were really nice screens and form-factors.
Meanwhile my Kobo Clara is just trucking along with no obvious change in battery life or responsiveness (despite the standard e-reader application processor with its power-sipping weediness).
I have a pretty old (going on eight years) iPad and it still works well enough to read an ebook. I mean, how much power do you need for that really?
Minutes to start and get the book open, and stuttering when doing things like choosing a book and turning pages was eventual my experience on all the devices: iPad Mini, Nexus 7, and a Samsung Tab 10.1. To be fair, someone else I know had an iPad Air for years and it was completely fine, so maybe they're the only tablets that actually work (or the Mini was a lemon and Androids just are bad?). Or maybe tablets are better since then: it's been a very long time since those devices went into a drawer, never to be used again.
Battery life.
It becomes very important when you are on a trip and can't easily charge your devices.
(Of course personal circumstances differ.)