That design introduces two kinds of overhead at runtime:

- You need a pointer to the allocator (presumably you’d want to leave room for types beyond arenas). That’s 8 bytes of extra size on every object.

- You need to dynamically dispatch on this pointer for every allocation. Dynamic dispatch is a lot cheaper than it used to be on modern architectures, but it’s a barrier for basic inlining, which is a big deal when alloc() for an arena is otherwise just a pointer bump.