RCM4200 AudioPlayer

Hi all, I am trying to use an RCM4200 for audio playback. I am currently using a module with an SPI interface. Theoretically with 10mb/s of bandwidth I should be able to do quite a few channels of audio. However even after modifying the SPI library to directly access XMEM, and also the UDP library to do the same (allowing my FIFO which is in xmem to be accessed directly without large block copies), I can’t even manage to do 16 bit 44100 ks/s. Has anyone managed to use these modules for audio. I am using the SPI at about 3.5mb/s, so that isn’t my bottleneck. I have implimented a FIFO of about 204800 Bytes, as a ring buffer in xmem. Audio in mp3 or pcm below about 700kb/s is ok.