As there is an exponential relationship between speed, engine power and fuel oil consumption. Slow steaming can reduce the amount of fuel required to operate a ship more than proportionately, resulting in equivalent reduced emissions. Longer trip durations however, mean longer journey times.

This can yield significantly reduced CO2 emissions although. A study found a speed reduction of 10% translates into 27% reduction in engine power requirements and in a 19% reduction in overall engine power to cover the same distance (as more time is needed to cover it). Lower speeds are more effective if design speeds of ships are brought down as well. The only potential adverse effect may result from the increase travel time that will affect the transport cost of some commodities that may decide the change for faster transport alternatives (e.g. road and rail).

Speed reductions of up to 30-50% (depending on ship type) compared to design speeds can reduce CO2 emissions of up to 40%.

In practice the implementation and decarbonisation potential of regulations has been limited:  Ship owners do not have the right incentives for choosing high-cost but highly efficient technologies and operational changes, and choose instead low-cost, low-gains designs. The market pushes for design improvements without the need for regulations, which has decreased the marginal advantage of regulated vs. non-regulated scenarios. Efficiency gains could also generate adverse effects, such as increased speeds, which could have negative CO2 mitigation consequences. Regulations for improving energy efficiency of new ships were initially seen as a having a high potential for maritime transport decarbonisation. In 2011, initial studies of future impacts of regulations estimated potential emissions decreases of up to 23% of CO2 emissions of shipping activity by 2030.

More recent estimations, however, put the contribution of regulations at only around 3% of CO2 emissions compared with scenarios without any regulation. The limited impact can be due to a number of reasons. Regulations, in particular EEDI ones, take time to be implemented since they mostly only affect new ships. At the same time, beyond regulatory effects, market incentives have seemingly pushed for fleet improvements quicker than regulations estimated or mandated. By 2016, a substantial share of the new build fleet already complied and over-complied with current and future (2025) design efficiency requirements. Notably, 71% of containerships, 69% of general cargo ships, 26% of tankers and 13% of gas carriers already comply with the 2025 EEDI requirement (i.e. -30% reduction compared to reference line).

Strengthening EEDI and SEEMP regulations could bring about additional emission mitigation gains. Putting in place EEDI a similar energy efficiency index for existing ships could more than double mitigation gains obtained from implementing the regulation exclusively on new ships by 2030, relative to 2013. Likewise, putting in place annual efficiency operational rations in SEEMP could be linked to potential CO2 reductions from 5 to up to 43% by 2030, also compared to 2013.