So it seems to be when using a display mode in a detail which has shadows and the skylight is on. This is a drawing of a steel structure with a bunch of nuts and bolts, so there are a lot of objects in the detail (92 block instances, 1 curve, 211 surfaces, 9902 polysurfaces, 671 extrusions, 176 meshes). If I change to a display mode with no shadows or turn off the skylight, plot speeds are back to what I would expect.
It is happening during what I would consider the spooling, where you get the preview of the sheet in green, and as each bucket is processed it clears the green. I am seeing this while printing vector, raster is faster, but the resulting sheet is blank when using a display mode with shadows and skylight. Rhino’s memory usage also greatly increases, going from around 4gb on file open to 8 - 16gb during the plot (just watching the task manager), so perhaps some of the slow down in paging? One detail on this sheet, no clipping planes for that detail (which is the usual speed culprit).