Topaz Video Enhance can create smooth slow-motion, slowed down by up to 2000%.
To give a comparison, I will slow down a clip with Optical Flow in Final Cut Pro to compare, using the Chronos model in Topaz Video Enhance with an 800% reduction in speed.
From a practical point of view, slowing down video clips stretches the space between the real frames of video.
Something needs to fill those gaps.