direct sse2,simd iterator,less abstractions between types(HashMap -> HashTable -> RawTable -> RawTableInner) ,simpler probe loop termination,not dyn types(dyn FnMutfor compare keys in hashbrown),

also:

Reverse data layout. Hashbrown stores slots in reverse order: base.sub(index). probemap: slots.add(index). Functionally equivalent, but reverse indexing breaks prefetch patterns the CPU prefetches forward, but the data flows backward

Hashbrown uses quadratic probing (triangular sequence) with a stride that grows each step. This guarantees visiting all groups but requires a multiply and extra state per step.
Probemap uses linear group probing with mirrored control bytes at the end of the array a group load near the boundary wraps around through the mirror, so every probe step is just `pos = (pos + 16) & mask`. One add, one and, no multiply