Odds are far better than good that your high performers are achieving what appears to be high levels of productivity by building technical debt into the application by taking shortcuts whether intentionally or unintentionally.
This can be applied to design as well as development, with similar consequences. Building in debt is a great way to get ahead, but only if everyone is aware they'll be circling back and spending 4x a long to fix it later. That's the thing with technical/design debt... It takes much longer to fix it later, than it does to implement a proper solution to begin with. I've seen many people, including myself, get burned by it.