Good article here... basically, the PSP is capable of running at up to 333 MHz, but Sony have purposely limited it to 222 MHz for the time being. In other words, the machine's only running at 2/3 of its rated speed. They're doing this to force more efficient coding from programmers, and because at 333 MHz, current battery technology would last only a couple of hours. At some point, they'll unlock the speed limit and PSP games should look even better than they do now (can you imagine Wipeout Pure with 50% more polygons?).