I was monitoring with iostat and a couple of other tools. It was certainly very heavy on io, with 80% util, 20% iowait, and that increased as the currency went up.
I was using protobuf, and a w value of 1, so I was out of things to optimize.
When I was inserting objects already in Riak's cache, it ran about 3 times faster, but of course that's not possible with new objects.