Despite decades of efficiency gains, aviation CO2eq emissions continue to rise because savings have been reinvested into lower fares and traffic growth. Existing decarbonization scenarios from ICAO and IATA rely on Sustainable Aviation Fuels (SAF) and incremental technology, while ignoring flight speed reduction. Using operational data from over 1,300 commercial flights, we compare jet and turboprop aircraft of similar capacity on equivalent routes and demonstrate that turboprops reduce fuel burn by 37% for a marginal time penalty. Turboprops cruise at lower altitudes, below the ice-supersaturated regions where persistent contrails form, delivering a double climate dividend. Integrated scenario modeling shows that combining speed reduction with SAF deployment reduces required SAF volumes by approximately 184 Mt per year by 2050. We propose reversing the historical speed–price paradigm as a prerequisite for credible aviation decarbonization.