This was on a 10 year old Xeon CPU.

Sqlite is a single compilation unit for much very different reasons than the header only libraries by the way. It's developed as many different files and concatenated together for distribution because the optimizer does better for it that way.

What difference does that make? What does it have to do with anything? You could develop anything like that.

The point here is that single file libraries work very well. If anything they end up making compilation times lower because they can be put into monolithic compilation units. When I say single file C libraries, I'm talking about single files that have a header switch to make them header declarations or full functions implementations. I'm not sure what you're trying to say.