What Janne said: you're IO bound, not CPU bound. Launch top (or better, atop/htop), notice how little CPU is actually used when transferring small files. Also note that your processes are in 'D' state, waiting for data to be available to them.
Additionally, I don't believe rsync is optimized for multi-core; most of what it does is sequential, and it would require very clever work to make it go faster in that respect.
It does, however, probably take advantage of up to 2 cores if you use ssh as a transport. It will be spawned as a separate process, and will do all its encryption and possibly compression work in a separate thread from the main rsync process. Said process has somewhat CPU-intensive tasks: CRC calculation and MD5 hashing (I believe that's what it uses).