Custom datastores (custom at the framework level, not necessarily at the application level) have been very useful for me in the past for getting high performance.

For example, I wrote a little in-memory DB for use in a .NET framework to avoid having to deserialize an object graph per request. Instead, it navigated a byte array and plucked out just the data needed.

Another time, I wrote a compact in-memory representation of a trie, using bit-packing and a few other tricks, to get an order of magnitude reduction in memory usage, making it possible to cache a lot more of a data set.

Why go to the database and instantiate a classic object if you can avoid it?