Traditionally Iowa Blues are fast feathering birds, and hicks hatched from the traditional lines have a fast feather growth.
In my experience and in the experience of many others, birds obtained through the various hatcheries often express the slow feathering. So, it really depends on where stock is obtained as to the speed of feathering.
Speaking from a club standpoint, we encourage breeders to breed away from slow feathering mainly because-
1) Traditionally the breed had fast feather growth
2) We want our hens who hatch out chicks to be able to raise those chicks up. When the grass is full of dew and the hens are running around with their chicks, the faster the chicks grow feathers the easier it is to keep them alive. During the first week of life the hen will not move her chicks too much, but by the end of the first couple of weeks, she's moving them all over the yard/pasture. If they feather slow, then when she starts to get active in moving the chicks, they'll get wet from the dew, chill, and die pretty fast. In order to prevent this, we need to have our chicks with some decent feather growth by the end of a couple of weeks. Mind you, they won't be fully feathered, but their wings should be long, their tail feathers long, and some feathers starting on their breasts and shoulders. This makes a drastic difference in the chicks survivability.
I haven't experienced the longer feathers female/shorter feathers male within the traditional Iowa Blue lines. I have definitely experienced this with many other breeds, but not here. I did notice this trait to some extent in the commercial or hatchery lines, though not always a reliable indicator of sex. While most of the males grew feathers slower, a few females would too. And vice versa. Others will need to chime in and share their experiences concerning this.
Hope this helps answer some questions.
