Microsoft didn't fail at IronPython and IronRuby for technical or perf reasons, they owned the projects and essentially withdrew funding, presumably this delayed all the dlr work too dynamic invoke etc.. rosyln changes etc.., it was a disappointement at the time to me 2010? and essentially allowed the JVM to pull ahead in community adoption of other languages. Where MS is now this decision to withdraw from iron* looks insane, but the attitude to open source and community was different then. Time has pasted. Happy clojure developer on JVM today