If speed is the overriding concern, see if you can use Berkeley DB, which has ACID transactions and blows away all relational databases in speed. It sounds like

If speed is the overriding concern, see if you can use Berkeley DB, which has ACID transactions and blows away all relational databases in speed. It sounds like your data model may be simple enough for it. You should also consider a mix of both; you could use BDB as a kind of cache used for rapid delivery whereas, say, the relational database could be used for non-time-critical processing and storage or something.