I remember applying a blur effect in Photodesk 3 on my Kinetic RiscPC. This took around an hour. One hour!!! Absolutely ludicrous (at the time, Kinetic was the top machine). For graphics work and the like, the more speed the better. Plus as has been mentioned, powerful hardware and a fun OS are what attract developers and users.
If it was feasible, porting to x86 hardware would make sound commercial sense for the desktop, as it offers RISC OS on cheap but powerful hardware. Develop one version of RISC OS for x86 desktop use and one for embedded/portable ARM-based devices.
But back to reality, I'd like to see faster ARM processors coupled with more powerful graphics hardware. Perhaps dual-processor boxes. Something powerful enough to encourage some software development, such as new bitmap editors, video editors etc.