35 tons for 4 hours...
From Boeing website:
Up to 15 containers or pallets, each measuring 88 by 125 inches (223 by 317 centimeters) at the base, can be accommodated on the main deck of the 757F. Total main-deck container volume is 6,600 cubic feet (187 cubic meters) and the two lower holds of the airplane provide 1,830 cubic feet (51.8 cubic meters) for bulk loading. These provide a combined maximum revenue payload capability of 87,700 pounds (39,780 kilograms) including container weight. When carrying the maximum load, the 757F has a range of about 2,900 nautical miles (5,371 kilometers).
That makes it 725 knots/hr!