Rockboy's performance is less than stellar on the PortalPlayer targets (iPod, iriver H10, and Sansa). There hasn't been a lot of work on Rockboy by the developers because they're prioritizing on other parts of Rockbox that need more work done.
As I explained in an earlier post, the speed you get depends on what you play as well. An early Game Boy game (i.e. Alleyway, Tetris) will give you fairly good performance; whereas a Game Boy Color Game (i.e. any of them) will run slower than Slowpoke Rodriguez (the slowest mouse in all Mexico

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If you're good at C and assembly programming, you can download the source for Rockbox and maybe help optimize the plugin.