Dunno. I play one voice of IRCAM Multi Granular with 8 grains and it's roughly up to 8% CPU and as low as 5.5% CPU (as reported by Task Manager) over here on my i7-6700K all core OC'd to 4.5 GHz... So yeah you don't really want to play chords with it.
Nah I'm pretty sure it's not the mobo at all. That's extremely rarely a factor in these things.
But for reference, it's AsRock Z170 Extreme 4.
Unsure. You only mentioned you load one sample in, but this is far from the whole picture. How many multi granular voices, and do you play it polyphonically then? This is where the CPU usage starts multiplying.