To be honest, Zuma hardly looks like a complex game and my guess is it's not taxing on the CPU.
Rockdoom, on the other hand, uses a 3D engine and it is quite CPU-intensive. Similarly, Rockboy is an emulator which is very CPU-intensive. The lack of knowledge about the video chip is definitely an issue along with the iPod having a slow CPU.
If you're looking for pretty games, try Jewels or Bubble or Brickmania. Remember, you can always provide new art for the games and try to get the graphics changed in SVN.