I've experienced similar things, but my conclusion has usually been that the model is not receiving enough context in such cases. I don't know your specific example, but in general it may not be incorrect to put an Arc/Lock on many things at once (or using Arc isntead of Rc, etc) if your future plans are parallelize several parts of your codebase. The model just doesn't know what your future plans are, and in errs on the side of "overengineering" solutions for all kinds of future possibilities. I found that this is a bias that these models tend to have, many times their code is overengineered for features I will never need and I need to tell them to simplify - but that's expected. How would the model know what I do and don't need in the future without me giving all the right context?

The same thing is true for tests. I found their tests to be massively overengineered, but that's easily fixed by telling them to adopt the testing style from the rest of the codebase.