Swift's unpredictable efficiency

I don't know: it's a working example of the error and the crazy annotations, not of the performance issues.

Yes, but this whole thing started because I'm trying to implement a graph analytics library using public struct Graph<T: FixedWidthInteger>: SimpleGraph where T.Stride: SignedInteger and performance is horrible compared to when I incorporate the code directly into an application, which is specifically what I don't want to do (see Performance issues when using a library vs including in code). I confess that I have not had the patience to create an MWE for the performance issue.