Thanks, that clarified things. Unfortunately I too know little about actually writing compilers so my depth is limited. But I will offer some thoughts anyways.

Thanks, that clarified things. Unfortunately I too know little about actually writing compilers so my depth is limited. But I will offer some thoughts anyways. The slow down in dynamic languages is in more than type checks though. The slow down is because dynamic languages are like some crazy awesome dream world where anything can change at any given moment and things are not necessarily what they seem. So VMs must be extra vigilant, checking many things like assignment, for exceptions, if the object is still the same class, if it still has the same methods etc. The term dynamic is almost an understatement! Add to that boxing and the possibility of heterogeneous collections and slow downs are the price to pay for all that flexibility. This makes things very hard to predict both on the compiler level and also at the CPU level in terms of branching - which alone is very costly. Dynamic language programs are not easily compressed. There are ways around that and languages like Clojure offer a sort of compromise, by being able to lock certain parts down in a solid reality, structures (in the sense of regularities) coalesce and can be used to speed up parts of your code. You can choose where to trade flexibility for speed. You're right (Subtype) Polymorphsism do incur a cost but modern CPUs can well handle these. But with parametric polymorphism and value types you get no runtime hits and also get some free theorems to boot. That said, how you think has to have some influence. I have never found static types to be constraining, I actually feel like the allow me to more easily plan future consequences. I suppose I trade implementation freedom for the ability to create consequence trees of greater depth and quickly eliminate unproductive branches.