GWT achieves reasonable speed by only supporting a subset of Java which is already basically compatible with the JavaScript runtime. So, you don't get things l

GWT achieves reasonable speed by only supporting a subset of Java which is already basically compatible with the JavaScript runtime. So, you don't get things like threads, weak references, etc. To support such things (if it's even possible) would require writing (at least pieces of) a JVM in JavaScript. Want weak references? You now can not use JS GC -- you have to synthesize Java objects and write your own GC impl. Good luck getting that to perform.