[mikepurvis]

IMO it hurts developer productivity more than performance, because it introduces indirection and potentially unhelpful abstractions that can obscure what is actually going on and make it harder to understand the code.

In raw performance this could manifest as issues with data duplication bloating structures and resulting in cache misses, generic structures expressed in JSON being slower then a purpose-built struct, chasing pointers because of functions buried in polymorphic hierarchies. But I doubt that any of this would really matter in 99% of applications.