Sowing could go a lot faster because in 5-15th century period they used to throw the seeds by hand while slow walking along their field. Which would be faster then how it goes in game at this moment.
But a seed drill/mechanical was invented in a later time period the 16th century. And a simple
http://en.wikipedia.org/wiki/Planter_(farm_implement)
also seems to advanced for the time period. Although it could be added if you ask me. If the current planting/sewing method wont be changed. But only for crop that is planted by seed.