The notion of coherence in time-dependent dynamical systems refers to mobile sets that do not freely mix with thesurrounding regions in phase space. Coherent sets can be identified using set-oriented numerical methods involving transfer operators.Typically, such approach relies on the integration of a large number of particle trajectories computed from test points initialized inthe elements of a fine partition of phase space. This can be prohibitively expensive in complex flows involving turbulence. Here,we systematically study the robustness of such computations in an example system. We demonstrate that the transfer operator basedframework still gives reliable results even when both the number of partition elements and the number of test points are reducedsignificantly.