It isn't that He needs to meditate His reply. That part is obvious: they're daring Him to defy the Empire for the sake of perfect legality, so He'll dare the same of them. Simplicity itself.
But that only answers their challenge; the question itself touches much deeper. Moses commanded us to stone; what sayest Thou? Soon, He will answer that as well – and every fiber of His humanity trembles at the thought.
So, to steady Himself before replying, He bends over and writes in the dust – meaningless scribbles, a mere purchase of time, a formless jumble of noughts…
And crosses.