Is that with the js_binary/library rules or something new? Last time I checked, all the work that goes into compiling the JS sources and minifying them into a single "binary" blob happens in the js_binary definition, and the js_library definitions traditionally served only to construct a total ordering of raw source source file dependencies.
Two partial answers: there's a newer better thing thats now the default and splits the building, but even under old js_library, there were some gains from not globbing everything together...
