You don't have to set up local mirrors. You can start the build with periodically-saved node_packages, or if you're using docker "ADD packages.json, RUN npm install", or commit vendored modules, or do one of thousand of other things.
For some companies release just means another merge to master, so they are doing tens/hundreds releases a day.
Once you do enough development that this matters, you're going to be doing caching or you'll be wasting money on waiting and bandwidth.