[munificent]

> Modern JS engines won't use a hash table for the object state, they'll use a hidden class, and member accesses will be fixed offset indirect loads guarded by a type check.

The type checks themselves have significant overhead, and it's easier to fall off the shadow class fast path than you might expect.

> Initialize your objects carefully in a deterministic order, and I'd expect you can control field order and adjacency.

True, but that's equally true of non-ECS architectures. I have yet to see much evidence that the ECS JS engines I've looked at are actually taking that into account.