db4o is the open source object database that enables Java and .NET developers to store and retrieve any application object with only one line of code, eliminating the need to predefine or maintain a separate, rigid data model.
Prevayler is an object persistence library for Java. It is an implementation of the System Prevalence architectural style, in which data is kept hot in memory with changes journaled for system recovery.
Both db4o and Prevayler are fast and they actually let you deal with object directly, and thus make them very developer friendly. For RDBMS you need to either use OR mapper or SQL directly.
In my optiion, if performance is your number 1 priority and memory is not an issue, then you should consider using object database. An interesting study can be found at Open Source Database Benchmark.