I'm looking for more concrete data about this argument of Foundation being monolithic.
One data point that we have heard is that it brings in unwanted dependencies. We intend to address this via splitting the URLSession and XML pieces out (see Pitch: Move URLSession to new FoundationNetworking module).
With respect to compiler optimizations, we believe those can be applied to struct Data regardless of its position in either the stdlib or Foundation.
Is there something else that has a measurable impact that we can address?