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Lookup cost depends on collisions, and collisions depend on the hash function. Rust's standard map uses randomized seeding for collision resistance, while the exact hash algorithm is not a stable public guarantee. Swapping in a faster non-cryptographic hasher is correct for internal keys and dangerous for user input.
A hash map maps keys to buckets using a hash function and resolves collisions according to its implementation. Expected lookup can be close to constant time under suitable distribution and load, but a worst-case collision pattern can degrade performance. In Rust, the standard map uses randomized seeding to make attacker-controlled collision selection harder; do not rely on one fixed hash algorithm as a permanent API guarantee. Choosing a custom faster hasher may help trusted internal keys but can weaken resistance to adversarial inputs.
The choice of key type matters too. A large string costs time to hash and compare.
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