It started with the Pentium 4. The tradeoff of power/heat for the increased clock speed stopped paying off. In fairness, Apple/IBM/Motorola also learned this

It started with the Pentium 4. The tradeoff of power/heat for the increased clock speed stopped paying off. In fairness, Apple/IBM/Motorola also learned this lesson with the G5 but they were minor players in the market and were largely ignored. So it was roughly between 2005 and 2008 when perf/watt became a thing. I think public awareness of it had a lot to do with Steve Jobs relentlessly pushing perf/watt to help explain the PPC to Intel transition as Apple was just starting its public awareness rocket ride thanks to the iPod taking off.