I'd be curious how this stacks up on ANN-benchmarks (https://ann-benchmarks.com/).

FWIW exhaustive search is still probably good enough for most use cases. IMO the exhaustive search should use a heap https://github.com/fennel-ai/fann/blob/main/src/main.rs#L12 as you're only looking for top-k, it reduces time and memory complexity greatly. On a relatively unoptimized Golang implementation (though much beefier hardware) I get ~100ms to process 1M vectors of 300 dim. Still quite a bit slower than approximate, of course, but in absolute terms probably good enough for most use cases, especially because many use cases don't have 1M vectors.