>In my experience though projects tend to start in a single crate, without great attention to their internal dependency graph, and once compilation time becomes an issue, they have already created a spaghetti dependency graph that is difficult to refactor into smaller crates.

This happened to me too. One important problem to overcome is the orphan rule. This means: you can only implement a trait you own or for a type you own. If both the type and the trait are not in your crate, you are out of luck.

So my first attempt failed utterly. It was very disappointing. You see, by splitting the project in different crates I put one type and one trait in different crates.

After thinking about this problem for a few weeks I accepted that I needed to write wrappers, so called newtypes. But this hurt a lot. I managed to abstract away a lot of the boilerplate by writing macros but still am not very happy about the complexity.

Growing pains.