Computation time of one stray field computation for (a) different 2-D grids of N × N cells and (b) different grids of N × N × N cells. The red (blue) line shows the runtime on the CPU (GPU) configuration. The grey bars show the speed up, e.g. the ratio of the CPU time to the GPU time. [Gunnar Selke: Development and conceptual design of a micromagnetic simulator for high-performance computing]