I'm not a fan of dynamic typing, nor am I a compiler expert, but I'm pretty sure this is wrong for (at least) JS engines, which are JIT-compiled down to static machine code and which use automatic type deduction. Only when types on existing variables change all the time it would be a performance problem. If JS code is 10x slower then it's native equivalent it's probably because some slow-path is hit (too many objects created and destroyed, burdening the garbage collector, or too "dynamic code" which causes many recompiles at runtime, or code not to be compiled at all but running through the interpreter..).