Code is "compute heavy" (could equally be memory heavy or IOPs heavy) if it's deployed into many servers or "the cloud" and many instances of it are running serving a lot of requests to a lot of users.

Then the finance people start to notice how much you are paying for those servers and suddenly serving the same number of users with less hardware becomes very significant for the company's bottom line.

The other big one is reducing notable latency for users of your software.

But sometimes, you do end up writing that compute heavy piece of code. At that stage, you have to learn how to write your own native library :)

Speaking of which, I've written some Python modules in Rust using PyO3, its' a very agreeable experience.