node2nix VS hasql-interpolate

You may find something like node2nix helpful (https://github.com/svanderburg/node2nix). This converts your package.json into a Nix expression that can then be cached. You're right that it does require some setup and a bit of Nix knowledge but could yield significant benefits and take a good chunk out of that 20 minutes.

Another option might be to use pnpm instead of Yarn and cache your pnpm dependencies. pnpm actually works a bit like Nix in that it creates a pnpm-lock.yaml file with content-based hashes for the full package.json dependency tree. This enables it to quickly determine which parts of the dependency tree it needs to build and which are already available.

I got it from this github issue in the node2nix repo.

As Gerschtli commented, it may be possible to generate a Nix expression for @mozilla/readability using node2nix. Then get its node_modules path like this and set NODE_PATH to the value.

For node programs, you may be able to find the commands as just packages, or look at https://github.com/svanderburg/node2nix for creating the nix equivalent of a node-modules

One project cut their CI time from 18m to 3m: https://github.com/awkward-squad/hasql-interpolate/actions. I wonder who will see the biggest cut!

Note that when PRs merge to the default branch, their cache doesn't come with them. This is how GitHub Action's cache works, as a security measure. However: subsequent rebuilds will, and PRs off the default branch will too.