Webpacker in Rails 5 takes a long time to compile not that many files. How can I see what it's doing?

I should've RTFM:

If you want to use live code reloading, or you have enough JavaScript that on-demand compilation is too slow, you'll need to run ./bin/webpack-dev-server or ruby ./bin/webpack-dev-server. Windows users will need to run these commands in a terminal separate from bundle exec rails s. This process will watch for changes in the app/javascript/packs/*.js files and automatically reload the browser to match.

Running ./bin/webpack-dev-server uses live code reloading and is super fast!

rails webpacker:compile essentially just runs bin/webpack. See compiler.rb#L59. Unfortunately you can't pass it any options via rake but you can run it yourself in verbose mode to see what's going on:

bin/webpack --verbose

That's a bit hard to read and doesn't give good profiling information. You'll probably want to use the --profile flag instead:

bin/webpack --profile

This shows you the time it took to compile each of your packs and how large different chunks of code are.

Edit: I notice you said webpack-dev-server solves your problem in another answer. It may for now but our application takes 7 extra minutes to deploy because our JS is so bloated. I'm working on trimming things down and knowing the profiling information for each pack is a necessity to lowering our deploy times.