Abstract The influence of the mass operators on the accuracy, economy and computational efficiency of the time-split group finite element formulation is investigated for the viscous flow over a backward-facing step. On a coarse grid it is found that mass operators must be retained adjacent to the computational boundaries to obtain the correct steady-state solution. The present time-split (or approximate factorisation) group finite element formulation is only 18% less economical than a fully lumped (equivalent to a finite difference) formulation. If mass lumping is introduced in the interior only, the solution is almost as accurate as if the full mass operators are retained, but there is little gain in economy in two dimensions. An operation-count estimate indicates that interior mass lumping would be about 40% more economical than retaining the mass operators in three dimensions.