alt Hacker NewsThat's the reason why polymorphism is sometimes described as slow. It's not really slow... But it prevents inlining and therefore always is a function call as opposed to sometimes no function call. It's not the polymorphism is slow. It's that alternatives can sometimes compile to zero
On the other hand, if the compiler can prove at compile-time what type the object must have at run-time, it can eliminate the dynamic dispatch and effectively re-enable inlining.