What impact does the metric have on performance? For instance, is computing cosine similarity generally more or less efficient than Euclidean, or is it roughly the same after transformations?

The choice between cosine similarity and Euclidean distance impacts computational efficiency based on the context and data format. Both metrics have a linear time complexity (O(n) for n-dimensional vectors), but their actual performance depends on implementation details and data characteristics. Cosine similarity calculates the dot product divided by the product of vector magnitudes, while Euclidean distance computes the square root of the sum of squared differences. For raw, non-normalized vectors, Euclidean distance often requires fewer operations per dimension because it skips the division step. However, if vectors are pre-normalized (e.g., converted to unit vectors), cosine similarity simplifies to a dot product, making it as efficient as Euclidean distance. In practice, the difference in raw speed is often negligible for small datasets, but it becomes noticeable at scale.

Transformations like normalization can equalize the efficiency of the two metrics. For example, if vectors are normalized to unit length upfront, Euclidean distance between them becomes equivalent to sqrt(2 - 2*cosine_similarity), which requires the same number of operations as cosine similarity. This pre-processing step shifts the computational cost to an earlier stage (normalization), making subsequent comparisons faster. However, normalization adds overhead if done dynamically for each comparison. Developers must decide whether to prioritize upfront costs (e.g., normalizing during data ingestion) or per-query costs (e.g., computing magnitudes on the fly). For applications requiring frequent updates (e.g., real-time systems), skipping normalization and using Euclidean distance might be more efficient unless cosine similarity’s scale-invariant properties are critical.

Specific scenarios highlight performance tradeoffs. In sparse data (e.g., text TF-IDF vectors), cosine similarity benefits from optimizations: non-zero elements dominate computations, and precomputed magnitudes reduce per-query costs. Euclidean distance can also leverage sparsity by ignoring dimensions where both vectors are zero, but the square root operation adds minor overhead. For dense vectors (e.g., image embeddings), both metrics perform similarly after normalization. A practical example is recommendation systems: if user/item vectors are static, pre-normalizing them optimizes cosine similarity. In contrast, Euclidean is preferable for dynamic data where normalization isn’t feasible. Ultimately, the choice hinges on data properties, use case requirements, and whether pre-processing is viable.