High CPU when DSD playback on RPi2

Hi , All,

I met a problem with my setting up.
I was running 1.5 over RPi 2 with a Durio Sound PRO. The source is a NAS running WHS2011 for SMB connected through WIFI.
192k flac is perfect and sound is fantastic.
But when I play DSD noticeable hiccups appeared especially for higher rate ones.
I checked and the bandwidth should not be a issue as I tried over USB storage as alternative to WIFI but the CPU was always very high.
For flac, the CPU runs at 30-70%
For DSD64, 80-100% the quality is just acceptable most of the time.
For DSD128, constantly over 100% and dramatic hiccups making it unacceptable.

Anyone has an idea how this happened?
What does the CPU do when it process the DSD data? Shouldn’t the decoding job done by the DAC?
BTW, only DoP worked and straight playback gave no sound at all to me.