Khan didn't plan on wasting time in that environment but couldn't help himself at that sight. He recognized the object that had sealed the cavity, and his senses soon became able to study its insides.
It seemed the flexible membrane had enhanced the Great Old One's ability to mask its aura, preventing Khan from perceiving its insides. That, or the Great Old One had stopped hiding, allowing Khan to sense the entirety of its existence and the might it carried.
The lake had shown humanoid giants, but the membrane contained nothing remotely human. The victor of that ancient war had been tiny in comparison, but its presence was nothing short of colossal.
Khan didn't think everything through, and facing that unthinkable reality inevitably led to hesitation. He didn't change his mind, but his brain needed time to absorb that gargantuan scene, time the Great Old One didn't give to him.