Comment by OskarS

I would bet that if you compare it to a traditional server-client database (which functionally does essentially the same thing: you send it a query over the network, and get a result back), the overhead is probably massive. This is a very clever way to cram that kind of functionality into a static hosting site, and you can imagine some uses for it, but it's clearly not a particularly efficient compared to doing it the "right" way.