out3d_vars lets one disable output of buffer regions and has to use the "active" region for this. Right now it recomputes the active region for each variable each time output is requested. This can be slow for large MPI rank counts. It is not necessary to recompute it however as the required information is stored as part of Carpet's
level_boxes data structure.
For large rank counts (~10k or so) this can take many minutes to recompute if many variables are output.