I use a pocketbook right now, and was using quite a few kobos over the years prior. At least with those, I've noticed that we've kinda reached some ceiling for responsiveness, and I think it's the software/computing hardware not the screen hardware causing it. Stuff like page turns can be quite fast, but closing out of a book and opening another really feels like you're straining the poor thing.
Pocketbook particularly takes ages to reflow text if you rotate it. I think it's reflowing the entire ebook to get page numbers + chapter positions? Very annoying if you forget to turn off the accelerometer.
Try using KOReader instead of the built-in reader software. It's a bit faster (doesn't wait to reflow the entire book if you change fonts, margins, orientation, whatever), and also it's easier to achieve consistent rendering of books (fonts, spacing, etc.) as it supports overriding the stylesheet, and just generally handles HTML/CSS better.
Oh, and speaking of responsiveness, I found that it depends on temperature a lot. E-ink apparently has a sweet spot at room temperature, but when I use the device outdoors, in either sub 10 ˚C temperatures or over 30 ˚C the screen changes noticeably slower.
Pocketbook particularly takes ages to reflow text if you rotate it. I think it's reflowing the entire ebook to get page numbers + chapter positions? Very annoying if you forget to turn off the accelerometer.