Either keyframe or cut the part you want slowed down. Cutting it in a certain spot does the same thing as keyframing the speed unless you take it a step further and mess with the tabs on the keyframe then you can decide when you want the slow down to start occurring and makes it look a little smoother. If you don't care about it just cut up the clip and slow that one part down, to me both look fine as long as it is intentional and doesn't seem out of place.