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?
In any case, the way I read it, the requirement doesn't mean you have to make anything particularly heavy, just that you need to use a representation that's general enough to be usable in other scenarios without rewriting the entire app.
Of course, a little bit of extra encouragement in the form of a benchmark that's slightly harder to game wouldn't be bad.