The secondary hash must primarily be good at mapping the hashed value into a slot: that is, be as cheap as possible (ideally 0 cycles, fib hashing is ~5 cycles)

The secondary hash must primarily be good at mapping the hashed value into a slot: that is, be as cheap as possible (ideally 0 cycles, fib hashing is ~5 cycles) if the primary hash was good while preventing catastrophic performance if the primary hash was bad. All of this without knowing anything about the primary hash.