Yep. If you parallelize with goroutines it'll be faster, but we're talking about shaving a few ms. The effectiveness of a for loop crunching floats in a single thread is not to be underestimated.
FWIW - Weaviate's pure Go implementation of cosine similarity is probably as fast as you can possibly get in pure go, without SIMD -- https://github.com/weaviate/weaviate/blob/2f44dbee52196656bd...