It seems the Baltic was averaging about 9 knots an hour when she was reported off of New Jersey on the 9th. This would place her at Charleston at around 2.30 a.m. on the 12th. The Nashville was averaging about 8 knots an hour when she was reported off of Cape Fear, N.C. on the 11th. That is a difference in speed time of about 9 hours. That explains why the Baltic got there so much faster than the Nashville.
Both ships average normal speed was about 12.5 knots when running the mail in normal weather.