Although Neutrino is a realtime operating system, this fact alone doesn't necessarily mean that any given application will run quickly. Graphical user interface applications can consume a reasonable amount of CPU and are particularly sensitive to the end-user's perception of speed.
If at all possible, try to prototype the system on either a standard PC (in the case of x86-based designs) or a supported evaluation board (in the case of x86, PPC, ARM, SH, and MIPS designs). This will very quickly give you a "feel" for the speed of a particular processor.