The plan is to add up front filtering (i.e. filtering in the master node, not in the workers) to ALL calculators. I am not doing it right now because this pull request wants to be minimal, but adding the filtering makes a lot of sense. For one, it save a lot memory if somebody wants to run a computation with a big source model (and few sites) on a laptop. However, supporting up front filtering for thousands of sites is not trivial performance-wise, this is why I do not want to do that now.