Some lenses, like macro lenses, have a longer focus throw, so the camera motor has more winding to do , increasing focus time. Then, some lenses don't have a large opening (does 5.6 mean something) so the focusing sensor take longer to figure out a focusing solution. Then, you have some lenses with deficiencies that have poor contrast, so AF has a hard time finding the proper focus. Then, there is THE perfect lens (f:1, short focus throw, super high contrast (does it exist)) where the reaction time of the camera would be almost instantaneous.