Yes, you're going to notice responsiveness issues when coming from a Kindle, especially if that's something you're sensitive to to begin with. That said, coming from a K3, which was far from a speed demon, that seems a wee bit harsh nonetheless

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I perfectly understand your PoV, so this may be beating a dead horse, but, do try KOReader if you want to see how the hardware *could* theoretically behave (Hello, menus that actually open on tap! Hello, page turns that effectively take < 100ms and actually barely make a dent in the CPU).
Otherwise, yeah, some stuff might feel slightly less than snappy. Some of it is somewhat outside of Kobo's hands (f.g., Adobe RMSDK's performance is atrocious on Kobos, for some reason, even if there has been some progress on this front, so, prefer KePubs over plain ePubs; and relying on Qt5 takes a heavy toll because it's really not tailored towards grayscale pixel formats).