As people have mentioned, on the Kobos, it also highly depends on how messy the (k)ePub code is, but, yes, in general picking up a PW2 after having used an H2O daily fills my heart with sadness at how much snappier it [the PW2] is. (Granted, the PW2 has a much snappier CPU to begin with, a fairer comparison would be against a PW1... which still is noticeably snappier, at the very least in the reader itself).
Also keep in mind that the mobi format is much more well suited to running on embedded devices with limited resources, which surely helps.
/methinks the IR-grid vs. capacitive touchscreen also adds another handicap to the Kobos (except maybe the Aura, since that one used a capacitive, too, AFAIR?).
Putting my H2O in the hands of people used to Sony & Bookeen readers also left them thoroughly unimpressed with its speed, FWIW.
As usual, YMMV, Pros and Cons on each side, yadda yadda
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