Hard to give specific advice without seeing your map, but assuming you only need raster for the parts with mosaics on them, then it would probably be much faster to export those mosaics separately (from the mosaic edit window), as png image files, with the "split into tiles" option checked. Make sure the resolution you choose is high enough for your finished AT5 raster (8 pixels per metre minimum for 0.125m pixel size). Once you have your separate mosaic raster tiles, you can run the raster process on the IMC by hand. This skips the export step in RM, and means you can update individual mosaics as required without re-exporting the rest. You export your map in vector only from the project, and combine with the raster files on the card.