That's interesting. I wonder what it is about high key profile that makes typing slower for you? If the board surround was high too and flush with the keys on a standard Cherry MX, would that hypothetical board make a difference?
I'm basically wondering the same thing. Across the field of keys, I don't see how the height of the keys makes a difference. Perhaps there's an ergonomic difference, as I can see a low profile board being easier to type on without a wrist rest. Conversely, I can see some strain developing if you're accustomed to typing at a lower angle. Otherwise, it could be the curve or lack-thereof that makes a difference, since most/all low profile boards I've seen have a flat field of keys (e.g. like a laptop.)
I know it took me a little while to acclimate to my Filco. I didn't type any slower, but the feel was obviously much different that my old MS Natural.