"Abstract early" is just plain bad advice because you usually don't know the full problem up front. Better advice would be to write code that lends itself resilient to refactoring (i.e. it is easy to refactor and won't break in unexpected ways when you do so)
Additionally, the authors "better" design now abstracts the primary performance problem out of sight of the developer. The main problem I see is that they are making multiple passes through the data to extract different metrics. By abstracting early they miss the opportunity to improve the performance by calculating all the metrics in one pass. The new design has hidden the root cause of the problem and prevented a developer from fixing the core problem without chucking out the entire design.
Additionally, the authors "better" design now abstracts the primary performance problem out of sight of the developer. The main problem I see is that they are making multiple passes through the data to extract different metrics. By abstracting early they miss the opportunity to improve the performance by calculating all the metrics in one pass. The new design has hidden the root cause of the problem and prevented a developer from fixing the core problem without chucking out the entire design.