ISTM that you think that this has much to do with CPU performance. in the case of the Iyonix and A6, it is probably the graphics card performance that is more significant (both use "PC" graphics cards).. The RPC is much more dependent on the CPU for graphics performance.
PC Graphics cards are optimesd to work well at typical PC resolutions and colour depths. These have long left things like 800 x 600 in 8 bit colour behind, and if they can do it at all probably operate quite inefficiently in these modes.
A fairer comparison would be somthing like 1280x1024 in 24 bit colour. then the RPC would struggle, if it can do it at all, Since the CPU has to do all the work. The Iyo and A6 would do much better, since they offload the work to a grahpics card that is designed to handle these resolutions.
This just goes to show the false picture that benchmarks give. Nobody with an Iyonix would choose to work as a rule in 8 bit colour, although those with older RISC OS machines might well do so. Andrew says, 16 bit colour works best on VRPC, but I think it would depend on which graphics card was used on the PC.
With the Iyonix my default mode is 1920 x 1440 in 24 bit colour. The RPC cannot do that. I don't know about the A6. But is is very usable on the Iyonix.
I think different VRPC systems with different graphics cards will perform differnetly, according to the performance of their graphics cards in whatever resolution and colour depth is chosen. It is much less dependent on CPU speed.