The only thing I can think of, is that for typical applications, memory speed is far more important. The other thing confusing me, is that I was of the impression that the IPaq used 24 bit and then converted to 12 bit at the last moment, in hardware. By the way, I don't claim to be an expert in any of this, so feel free to enlighten me. Thankfully it has flash ROM so a driver replacement is possible, if they do one down the line. Obviously Casio did their homework and optimized their video drivers for the Cassiopeia line.
Performance is a hard thing to judge.
Different apps respond differently depending upon what they use the most. It all depends on what the apps is doing. Well all have to wait and see. I've compared a 115 and a Jornada 548 side by side running through the same apps and the Casio is much faster in my opinion. I've got a few jimmy games loaded on it too and those are impressive.