Not using the same chip, at all. The Clara and the Forma share the latest e-ink controller used by the Oasis 2 and the PW4, and it does noticeably help them close the gap with Kindles, at least as far as how the screen is driven, which is kind of important
.
(The Oasis 2 did move to an i.MX7D, unlike the Forma. I have no idea how well that setup is exploited by the Kindle though, except in a generic sense that the Kindle software would be much more multi-core friendly than the Kobo's. But the 7D is a weird setup in that regard, so, eh. The "big" A7 processor in the combo is generally slower than the A9 we're used to, but it's potentially clocked higher, which possibly counteracts that... I wouldn't obsess too much over it, though.).
That doesn't negate the recommendation to keep to KePubs, because otherwise, yes, you'll feel the difference coming from a Kindle.
A good benchmark is now KOReader in the default Portrait orientation, since I unified how we behave on Kobo to match what the Kindles do, because it's best for performance. And despite that, the H2O still feels worse than my old PW2, just because the screen updates are jankier. On the other hand, the Forma is perfectly snappy, and that despite only being marginally faster in terms of processing speed (by that I mean a couple of milliseconds of difference, despite a higher pixel count, which does mean it is actually faster than the H2O on the processing front, despite using a similar CPU).