Nope, that is a restriction of the KDX, not the software (well, technically, it is the e-ink controller, but let's not dive into that). rdesktop won't be no different here. The KDX is simply pretty slow on updating the screen.
The only way to go faster is to use the dither mode of kvncviewer. But that would leave you with, well, a dithered b/w picture - not nice to look at for most applications (though not all!). The e-ink controller is faster for refreshes that are b/w only (no grayscale).
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