At the time Bach was composing, sopranos were always little boys or castratti. Women were not allowed to perform in public. Boy sopranos have always been popular, they still are. There is a beautiful innocence and purity about a male soprano voice. I don't think the slowness of the piece had anything to do with the singers. Being the epitome of Classical music, Bach's work is precise, intricate and takes a lot of technical expertise to play or sing correctly. I'm sure he had every intention of creating a slow, deliberate piece. If he had written something in a faster vein, the musicians would have practiced until they sang or played it the way he wrote it.