simo: what do you base your justifications on? Please, some references and not guesses.
"people are saying the Sharp Zaurus is too slow". But the Zaurus et al run WinCE/Linux, not RISC OS. In any case, your presumption avoids the issue that their are a variety of XScale processors with associated architectures.
I suggests that this ARM9 device runs in practice around the same speed as a PXA255 devices, and probably noticeably faster in many instances than a RiscPC (having faster bus, memory, etc).