The whole point of VoltDB is that it is (supposed to be) dramatically faster than in-memory MySQL. That's not too hard to imagine, considering that MySQL uses one machine and VoltDB uses a cluster.

In fact, the whole point of NewSQL in general is that it is possible to implement SQL and ACID on a cluster and be dramatically faster than MySQL. While NoSQL is high-performance non-ACID non-SQL on a cluster, NewSQL is high-performance ACID SQL on a cluster; they're not the same thing.