Use the Orphaned Data option to remove unused DGN linestyles from the current drawing.
In some circumstances, importing DGN files can result in a huge drawing file due to a large number of orphaned DGN linestyles.
This data can spread to unaffected files if data from the affected drawing file is copied to an another drawing file. You might also encounter other issues such as a lagging cursor when selecting annotations like text and dimensions, copy-paste not working, and drawing file taking too long to open.