Obviously he can't use Kamui on his whole body as they've shown that that takes a lot of time...but couldn't he just move the parts of his body that it could hit (presumably the torso as Madara would be dumb to risk hitting Obito in the eye because of the Rinnegan)? We've seen him do this in essentially an instant in both his fight with Minato and with Naruto, Kakashi, Bushy Brows & Bee (I'd post pictures but I still can't do that in posts for some reason). I just feel like Obito forgot all about how broken his Kamui is right after the mask finally came off.