That's not exactly it. It's more of a "If you have nothing that takes more than 1% of your resources, no single optimisation can get you more than a 1% reduction in your resources". That seem to be how most web apps are: you parse a little bit of HTTP, a little bit of JSON, you validate a few things, you call the database, that does a few things too, you have a bit of business logic, you call the database again, then have a bit of glue code here and there, and finally respond to the user with a little bit of HTTP and maybe some HTML, maybe some JSON.

If that's how your app works and nothing can be optimised significantly, that's usually here where you can make big gains in performance by changing a big thing. One of these big things might be to put a cache in front of it, because a cache hit will be way faster than responding again to the same request. Another could be to change language. For example, from Python to Go. Since Go is (most of the time) a bit faster on everything, you end up being faster everywhere. Or even from Python to PyPy, a faster implementation. Another could be redesigning your program so that you have one single obvious hot path, and then optimising that.

That seem to be the case for them here: no component is taking all of the resources, but by using memo everywhere, all of them take less resources, which leads to a good reduction of resources in general.