The average performance of hashtables generally depends on collisions being rare, but if the hashed index is trivially derivable from the key, then an attacker

The average performance of hashtables generally depends on collisions being rare, but if the hashed index is trivially derivable from the key, then an attacker can supply data where everything hashes to the same index, regressing performance to the point that it's a DoS attack. This is why cryptographically secure short-output hash functions like SipHash were developed.