Very few spells have a perceptible delay between casting and resolving. Modern magical theory has parallelized processes and widened bottlenecks that early wizards confidently proclaimed beyond the realm of optimization. Nowadays, barring exceptional circumstances, a spell that takes longer than about one second to activate is almost certainly going to backfire spectacularly.
This spell has been percolating in its caster's spirit circuits for two minutes and seventeen seconds, far too long for it to be safely deactivated, and is now on the verge of completion. The outcome has been polarized to smooth success or dramatic failure, and at this point its out of anyone's hands.