In theory, if it's all in a single application's memory and using langauge-native data structures, then you have a lot of power to tailor the data model to meet

In theory, if it's all in a single application's memory and using langauge-native data structures, then you have a lot of power to tailor the data model to meet your application's needs. Barring concerns like object overhead (which I realize is a major consideration in languages like JavaScript and Clojure), you should be able to come up with something that is more efficient than a general-purpose solution like a database without too much trouble. It's really just a special case of the Chuck Moore style, "I coded my own $thing in 1/10 the code and 10x the performance of the popular off-the-shelf option, in a week. NBD, it's easy if you simply choose not to implement any of the features you don't need." thing. Where DBMSes truly shine is working efficiently with data that you can't or don't want to keep in memory all at once, or for which lots of different clients need concurrent access. The first of those (but hopefully not the third) are absolutely considerations for certain kinds of browser apps, but maybe not something the parent was worrying about.