Bundle size
How much code a page makes the browser download. A browser then has to unpack it, read it and run it, so on a cheap phone a big bundle costs time long after the download has finished.
Adding a package is one line for an agent, and it won't notice it just added 200 KB for one date format. Nothing fails, because each addition looks harmless, and no one reviewing the diff sees the size at all. You need something to fail when it grows.
Say it to your agent
“Before adding a dependency, tell me how much it adds to the production bundle after minifying and compressing. Run size-limit and show me the figure. If it is over 10 KB, look for a smaller package or a few lines of our own first.”