Comparison of Cppsimu (open circles) and LOC (filled black symbols) run times for CO line calculations, using a 3D model discretised onto a regular Cartesian grid. The x-axis gives the linear model size N, the models having N³ cells, and the y-axis shows the total run time, including the simulation of the radiation field and the solving of the equilibrium equations. The figure also shows LOC run times when the ray-tracing on the Cartesian grid is made with the more complex routines needed for octree grids.