bucksboy: Due to the method used in emulation, interpreting each instruction as opposed to 'Just in Time' compiling, RPCemu and arcem run at a slower speed than VARPC. However you should rememeber that even VARPC started out in life as an interpreted system called Red Squirrel. The method's for HD access are not CPU bound and even interpretted systems will offer stagering speeds compared to native RO hardware. The 'disk acceleration', you talk about is nothing more than the underlying OS caching recent files in free memory, a feature that would provide a huge speed boost to native RO if Castle/ROL even get around to it.
As for Video Acceleration, VARPC/Red Squirrel use DirectX which provides a hardware independant way of using more advanced features of graphics cards (with software versions of the same code, if the hardware doesn't support it). A common accelerated feature that I believe is used in VARPC is Blitting, very rapidly copying a section of memory (representing the RPC's screen graphics) into the video cards VRAM, and thus the screen. Tom would be able to use DirectX himself if he wants similar graphics performance, but at the moment he's using the Allegro graphics library that has a few advantages too, including multi OS support and a simpler programming API.