...such that the search is so much faster and more flexible than any I've seen used by any of the airlines?

I would assume that they're not just throwing lots of server capacity at it, but that they are querying fare databases much more efficiently or something. To the point where in the amount of time to see the results for one day in any other search engine, I can see a month's worth of the result in Google Flights with the complicated criteria I want.

Very curious to know why they're able to do this...