My experience with the smoothbore has been quick kills. Often the deer has lain down within a few steps and then maybe kicked twice when he realized that something was wrong. But these were also fairly unpressured deer.
On the other hand I missed a nice 8 pointed a few weeks ago that my brother killed Monday. He started to take a step and I swung in front of him putting the .600 340gr roundball just in front of his chest. I thought that it was a good hit based on the reaction but when I looked around, there was no deer, no blood, no hair, and a ball gash on the ground a few yards past where he stood. Ah well.