JDBC doesn’t have to be slow. VoltDB actually caches ad-hoc SQL plans so even they run pretty fast these days. You can use JDBC and get good performance.

The biggest thing you lose with JDBC over the native VoltDB clients is better cluster awareness. In our native client you can get callbacks when servers fail, when backpressure is triggered, etc…

Now Hibernate over JDBC is another issue.

Edit: As merb points out JDBC is synchronous. This is true; if you have one client and one thread, this will be limiting fast. It's not a server-side issue though. If you have enough clients and/or enough threads, then you can still get very high throughput.