In general, a web app has to get pretty ridiculously bloated for javascript minification and so forth to impact launch time on a local file system. I'd call this premature optimization. (In my day job we're working on a pretty huge web app and we don't use any build process except -- just recently -- precompiling our LESS files (mainly to handle IE and mobile devices)). For us, the downside of a build-process to the debug cycle is a far bigger hit than the benefit of minification. (Perhaps we were overreacting to another project which used maven and had 2-3 minute builds...)
Of course if you're using coffeescript or clojurescript or whatever there's no avoiding some kind of build process.
It strikes me that automating and simplifying the build process is exactly the kind of thing you could wrap up in a pretty nice node-webkit application!