When an engineer takes a shortcut and scrimps on performance, they need to understand how that damages the user experience. Likewise, when a designer pushes an engineer to make a change that affects performance, that engineer should help the designer make the best overall design decision — not just roll over and do what the designer asked. It’s this type of respectful collaboration that makes great design happen.
Focusing on the core business is what differentiates real product design from interface design or even user experience design. Fundamental product design is really hard and requires a lot of legwork, but this is what designers at the highest level do — and it’s why their work is better than yours.