Often, tsunamis have been treated as a static problem. First studies demonstrated that for earthquake rupture velocities in the span of 1.5 km/s to 3 km/s, the kinematic and static part of the tsunami can be treated separately. The deformation generated by an earthquake is copied into the sea surface and then the tsunami is propagated. However, very slow earthquake rupture velocities in the span of 0.1 to 1 km/s have not been included into tsunami modeling. Here, we calculate tsunami efficiency, based on Kajiura’s definition, for different models. We demonstrate that rupture velocity cannot be neglected for very slow events, i.e, rupture velocities slower than 0.5 km/s. We calculate a relation between Magnitude, Rupture Velocity and Tsunami Amplitude to the Efficiency of very slow tsunamigenic earthquakes. Megathrust earthquakes (Mw >8.5 ) with very slow rupture velocity amplify energy from 10 to 60 times larger than moderate to large earthquakes.