This is just an analogy but in Swift String is such a commonly used hot path the type is designed to accommodate different backing representations in a performant way. The type has bits in its layout that indicate the backing storage. eg a constant string is just a pointer to the bytes in the binary and unless the String escapes or mutates incurs no heap allocation at all - it is just a stack allocation and a pointer.

Javascript implementations do their own magic since most objects aren't constantly mutating their prototypes or doing other fun things. They effectively fast-path property accesses and fallback if that assumption proves incorrect.

Couldn't python tag objects that don't need such dynamism (the vast majority) so it can take the fast path on them?