The "Build" section of project info in XCode offers lots of compiler settings. I'm seeing good improvements in performance (up to about 20%) when I choose the LLVM GCC 4.2 compiler with the "FASTEST-O3" setting.
Are there other settings that also improve performance when compiling for the iPhone?
If your are using lots of floating-point operations, it may be help to turn off compile for Thumb for the ARMv6 architecture.