More than that, from studying various VM techniques it seems to me, that most optimizations that make Javascript fast for computation actually harm performance

More than that, from studying various VM techniques it seems to me, that most optimizations that make Javascript fast for computation actually harm performance of calling into foreign code (like DOM) even more so because in most browsers where is often quite significant interface mismatch between JS VM and DOM code (like plain JS objects lying on separately managed heap from DOM objects).