Compilation with dependencies is *very* slow

[alexandergunnarson]

First of all, let me thank you for a job well done on Lumo! I'm really excited by the possibilities it brings to self-hosted compilation.

In my /src folder of a test project, I only have one file — let's call it simple.cljs — that looks like this:

(ns alexandergunnarson.simple)

(enable-console-print!)
(println "Hello world!")

I downloaded the Lumo source to experiment with and run lumo and then the following within the REPL:

(lumo.build.api/build "/Users/alexandergunnarson/Creating/Code/simple/src" 
  {:output-to "out/main.js" :optimizations :none :verbose true})

If I pass in no classpath args to lumo, calling time on the build shows it takes ~500ms — good! But if to the lumo REPL I pass in even one path (in this case it was "/Users/alexandergunnarson/.m2/repository/org/clojure/spec.alpha/0.1.143/spec.alpha-0.1.143.jar") it takes ~1100ms. This appears to increase linearly with the number of paths in the classpath, according to experiments I did with both passing in classpath to lumo and changing the classpath dynamically via lumo.classpath/add!/lumo.classpath/remove!. For instance, if I pass in the actual, full classpath of a real project of mine (not just this test project) while only putting simple.cljs in the src folder, Lumo takes ~350 seconds to compile it which is kind of insane. By comparison, with the same dependencies and same single source file, cljsbuild compiles it in a few seconds or less (the reason it's not sub-second appears to be due to startup time).

Additionally, when I run lumo.build.api/watch and make a small edit to simple.cljs, the same latency happens every time it tries to recompile. I would have assumed it would have leveraged some sort of caching strategy and/or change awareness à la Figwheel; were this the case I could even live with an initial 350 second compilation time as long as subsequent recompilations were truly incremental (a second or less for a small change high up in the dependency graph).

I would have expected the compiler to be dependency-aware and know that simple.cljs does not depend on anything, even though there are many files in the classpath.

Let me know your thoughts about this issue, and how I can contribute to push this forward!