Audience question reframed as: if JRuby does all this, why doesn't everyone use it? Charles answers with two or three recurring friction points: (1) startup time — historically painful (multi-second rake tasks), now ~400ms baseline and 1–1.5s for big loads; (2) missing C extensions — big ones (Nokogiri, JSON, Psych) have JRuby versions that auto-install with the gem, but many small C extensions still need a JRuby port or an API swap to an equivalent JVM library; (3) beyond those, once the app bundles it usually 'just works'.