Finally, achieving incremental compile within jars would be a massive boost as well: ie, compiling directly to output jars, and then supporting incremental compile that consumes the slightly modified jars. The JVM hates classpaths containing loose classes in directories (anecdotally, jarring all inputs improved cold-build performance by 4x in one case due to drastically fewer syscalls and less IO), and this definitely affects Zinc when lots of modules are in play.