Best time to make it through Washinton DC?

From Millville, I think getting to I-81 would add considerable miles and time. US-301 is a good choice. A little longer, but also a pleasant drive is US-13/113 down the Delmarva Peninsula across the Bay Bridge Tunnel and then US-13 to US-64 or US-264 to pick up I-95 near Rocky Mount. In my opinion, from Millville taking the Ferry would be a waste of time and money (and we use the ferry twice a year, but starting from near Cape May). Take the Delaware Memorial Bridge to I-95. Take the DE-1 exit. Switch over to US-13 if you want to avoid the tolls or stay on DE-1 to US-113.