I have been working on improving midiplayer the last couple of days and it has gotten a bit better for coldfire targets, the problems with the pp targets (which the sansa is an example) is that they play back midi in 44100Hz, twice the sampling rate of the coldfire targets, this is because either the hardware on those targets doesn't support 22050Hz or rockbox doesn't on those targets, this means that the pp targets have to do twice the work that the coldifre targets do to play midis.
I think utilizing the coprocessor on those targets could help performance a bit but as I don't own one I can't experiment with it.