Analysis of the FoM gain associated to different population control strategies and choices of the population importance ratio for the kinetic simulation of a single time step of the Flattop-Pu benchmark in its critical configuration. The comparison is repeated for three different choices of the time step size: Δt = 10⁻⁸ s, Δt = 10⁻⁹ s and Δt = 10⁻¹⁰ s. For each time step size all the computed FoM values are normalized by the FoM value of a reference simulation where Russian roulette is employed and no importance-sampling scheme is applied (R = 1). The results of the legacy algorithm are displayed in red, while those of the new algorithm are shown in black. The circle in the curve of the new algorithm represents the value of FoM gain associated with the importance ratio based on the solution of the adjoint point-kinetics equations.