Available with Standard or Advanced license.
Raster datasets referenced by the mosaic dataset item do not have pyramids.
Raster pyramids help the display performance and overview building performance of a mosaic dataset. Without pyramids on the source raster dataset, it may take longer to build overviews. If overview and pyramids do not exist, then it may take a long time to display your mosaic dataset.
Use the Build Pyramids and Statistics tool with the Build Pyramids and Include Source Datasets options checked on.