Evolution of particle rms speed in simulation starting with random motion of amplitude 1. Particles have mean-free-path λ = 0.1 and coefficient of restitution ϵ = 0.3. Drag forces are ignored. The blue line shows the results of a simulation with 100 particles per grid cell and full collision partner list, while the red and golden lines show the results of limiting the collision partners to 10 and 2, respectively, while increasing the collision probability accordingly. The results are extremely similar. The lower panel shows the instantaneous inverse code speed. Limiting the number of collision partners has increased the speed by a factor of approximately three.