Experimental data on the rapid (≈4.4 nm s⁻¹) axial growth rate of narrow (≈9 nm in radius) InAs nanowires (NWs) obtained by Au-catalyzed molecular beam epitaxy on Si substrates at a low surface temperature of 270 °C are presented. These NWs exhibit pure wurtzite crystal structure and an unusually high ratio of the average NW length over the effective thickness of deposited InAs of about 60 despite the presence of parasitic InAs islands on the Si substrate. These trends are explained within a dedicated growth model. In the absence of In evaporation, In atoms either diffuse from the substrate to the NW tips or remain in the parasitic layer. This leads to a linear time dependence of the NW length and other unusual growth properties that are thought to have not previously been accessed.