That's Clojure, it compiles .clj files on demand into jvm .class when you require them. Here probably his user.clj is loading the whole application. You can do some dynamic things to delay the loading/compiling until after your REPL starts, so that you get a repl in 1-2 seconds. https://gist.github.com/dustingetz/a16847701c5ad4a23b304881e... In production you would ahead-of-time compile.