The other comments hit some spots (although today with the use of bundling, I'm not sure caching matters as much as people make it out to), but something that is currently missing from the conversation: parsing performance. Not the speed that the browser fetches the .js file (which depends on network quality) but rather the speed the browser parses the actual code, which can sometimes become the bottleneck. Larger projects adds more code for the browser to parse, and that will make the initial page-load performance worse. Smaller scripts === faster page-load, which improves user-experience and all metrics related to that.