Of course running under Windows RISC OS is significantly slowed down. That is why it can only claim the performance of a RPC despite a factor of 10 advantage in processor speed. But how much of this is due to the emulator, and how much do to the overhead of runnin under Windows is not clear.
What I am really saying is that with VRPC running under Windows, it cannot be expected to perform any better than any other Windows app (although it is possible that some Windows apps are less efficiently coded than VRPC, and this will obviously affect the perceived performance). So the response of RISC OS under VRPC is limited by the performance of Windows as well as by that of the emulator.
Since the response of native RISC OS on an Iyonix appears better than that of native Windows, once suspects that it would take about a 6GHz processor before the responsiveness of VRPC will match that of the Iyonix.
Clearly the choice of the Windows platform for the emulator will suit those who also want to run Windows, as they can easily switch between RO and Windows without having to reboot the machine. Many might argue that the convenience of this outweights the (unquantified) gain in performace that would be achived by running a RO emulator under some other base operating system. In fact I doubt that there would be any advantage is that OS was Linux, but something more lightweight might make a difference. And as you say, porting VRPC to another OS would not be a trivial task.