(Harvested at 9 months)
Initially I didn't know why, but I've been researching it. I what I found was that sweet potatoes have a heat unit requirement. They need a certain number of warm days of suitable warmth to grow tubers.
Sweet potatoes are adapted to the warmth. And they prefer sandy, well drained soil. Plus give them a summer of hot days and warm nights and they'll produce really well. Thus I find my sweets produce better at my seed farm than at my homestead location, primarily because it's much warmer there. I've also noticed another trend. Although I grow them year around here, they produce more and bigger tubers when they are ready to harvest in the fall. Must have something to do with the shortening days.
From various university websites, I've learned that most sweet potatoes require 1200 heat units. Some varieties require more. A definition of a heat unit for sweets is.... "the daily 24 hour average temperature over 55°". So if my high for yesterday was 80° and the low 60°, then that day's heat units were 15 (80+60=140; 140/2=70; 70-55=15). But I often get days where the high is only 70°, low of 55°. Heat unit is 7. Thus the hotter my days are, the more heat units. (I don't get days over 90°. When it gets into the high 90s it actually slows tuber development according to the websites.) Based upon heat units, I can now see why my sweet potatoes grow way slower during the cooler half of the year. And this is most likely the reason my friend's sweet potatoes takes much longer to develop.
I was aware that peas and corn have heat unit requirements. Now I've learned that sweets do too. Something for me to keep in mind.