Source maps aren't downloaded on the user's browser until the dev tools are opened.

  • But there could be (minor) perf cost because there must be some extra stuff in the CSS (or JS file) that points to the correct source, no? So the bundled file would be larger, right?
  • Yes, but mostly the bundle will have only a few lines (one per source file?), so not a real perf cost. Still check exactly what it is.