[k_g_b_]

I think you're confusing nominal with static and structural with dynamic. This might be because there's few instances of more powerful static structural types implemented in common programming languages - OCaml's polymorphic variants, Typescript (partially dynamic). Very common structural types available in many statically typed programming languages are tuples.

The compiler can still know and optimize the data layout of any static structural type and tuples certainly are optimized in e.g. Rust. However, the flexibility of other structural types like polymorphic variants or set-theoretic types like unions mean that the data layout also needs to be more flexible and that comes with some overhead e.g. vs a nominal sum type (like in ML, Rust enums, etc) or struct/record.

Missing data layout optimizations comes mostly due to necessary uniform representation of dynamic types - the same as for nominal types - e.g. through runtime polymorphism language features as OCaml has by default, virtual methods or Rust's trait objects. Whether a structural type system allows such dynamic features (e.g. OCaml's row polymorphism) or not is a design question - I think there's still plenty of use for structural types in a language with only compile time polymorphism (monomorphization).

See also deredede's comment https://news.ycombinator.com/item?id=42191956