Since you are not using pre-determined projection, The loading will be a bit slower. However, You may like to build statistics before loading TIFF's into raster dataset, Also, Please don't build pyramids or use different compression since this would increase the timing. You can do that after everything is loaded.
Additionally, Have you tried using command prompt for loading data? (for ex. sderaster command) I have always found that command prompt reduces almost 50% time then using ArcCatalog. You can get more information here: http://links.esri.com/gsb-sde-admin-cmds-ref
Hope it Helps :)
