The problem arises when trying to step through time. Before the model can move forward in time, each node in space needs to exchange data with its nearest neighbours regarding ionic fluxes, blood volume, etc. In other words, all work at time t=1 needs to be completed before the simulation can progress to time t=2. Given that one simulation takes on the order of 100 million time steps, sending out 100 million tiny work batches, each of which needs to be returned in full before the next can be sent, is not a feasible solution.
It is a well known problem for distributed computing. You could send N times each workunit to workers so each slice should be returned at least once pretty fast. Although it could slow down overall progress but then you don't depend on slowest worker in your swarm.