C1158 Thousandyear Ice Prison

In fact, Hu Jiu'er's velocity upon shattering the Wind Spirit Bead became even more formidable than in her Nine-tailed Sky Fox form. The potency of the bead's wind element remained constant, yet Hu Jiu'er's human form was significantly lighter.

This explains why Hu Jiu'er didn't immediately transform into her Nine-tailed True Body