Its minimal on a decent spec machine.
On mine with 2x stereo streams via USB and 3x stereo via analog at 64 samples (2ms) it uses 7% host cpu while playing.
Cubase sx 3.1
Dual dualcore opteron 275s (quad cpu)
RME fireface
Oh this was using all 16 parts spread across the 5 stereo outputs. I also tried just using the USB outputs only with 8 sounds each, still it showed 7% host cpu.....very cool
ten