Fe II ejecta velocities of slow-evolving SLSNe (in colour) and general SLSNe samples (grey) at the time of maximum (top panel) and as a function of time (bottom panel). SN 2018ibb has a markedly low velocity at the time of maximum and a flat velocity evolution, which is in stark contrast to the bulk of the SLSN population. Its velocity at peak is similar to the slow-evolving SLSNe PS1-14bj and SN 2007bi. However, both comparison objects lack spectra at earlier and later times.