I don't know. Before reading Casey, I'd say most, if not all, of my APIs would look like the "clean code" style.

The process was "think about the model, design a class hierarchy that fits with the model, add the operations". Even if I don't think about extensibility, that's how I thought my code.

Why? Because when I think about performance, I used to think about algorithms and I/O optimizations (for example batching).

Now I'll look into this data-oriented programming-thing and see how that applies to my platform (embedded Java) :D