Grasped spinning is slow, therefore it would never have been used when production spinning was needed, such as in the middle ages.
If grasped spinning is slow or not is objective. People learned how to spin grasped from a very young age, they would have found it faster than a modern person who has spun using a different style for, say, ten years, pick up grasped spinning, try it once and find it slow.
Many people say Continental knitting is faster than English, however some of the world’s fastest knitters knit English style and English style knitting has been used for many years. All my cardigans as a small child were knit English style and my mother had to knitt them or I went cold. If only the fastest method was used in situations where people had to produce or go cold then why did my mother not use Continental or my Nana’s knitting machine?
If speed was the only factor, why was the wheel not embraced more readily? The wheel was considered to produce thread of poorer quality and it was hundreds of years after it’s introduction before it really gained hold.