We present flatspin, a novel simulator for systems of interacting mesoscopic spins on a lattice, also known as artificial spin ice (ASI). Our magnetic switching criteria enables ASI dynamics to be captured in a dipole model. Through GPU acceleration, flatspin can simulate realistic dynamics of millions of magnets within practical time frames. We demonstrate flatspin's versatility through the reproduction of a diverse set of established experimental results from the literature. In particular, magnetization details of "pinwheel" ASI during field-driven reversal have been reproduced, for the first time, by a dipole model. The simulation framework enables quick exploration and investigation of new ASI geometries and properties at unprecedented speeds.