Movie files contain video and audio information (called streams—but the name's not important).
When you drop a media file on it and choose a conversion type, Smart Converter immediately analyses your file and works out exactly what part of it needs to be converted to achieve the desired output.
In many cases neither the video nor the audio need converting. In some cases just one or the other need converting, and in a few cases both will need converting.
Doing this analysis ensures Smart Converter only converts stuff that needs converting, and leaves the rest alone. This has a two-fold advantage over many other converters; its usually very fast and maintains the highest possible quality of video and audio.
Here's a list of the four conversion speeds that you might see and what they mean:
Fastest - Neither the video nor the audio need converting. This will be super fast and retains full video and audio quality.
Fast - Only the audio needs converting. This will still be pretty fast and the video will retain full quality.
Slow - Only the video needs converting. Not so fast but the audio will retain full quality.
Slowest - Both video and audio need converting. This can take some time, especially for a full length movie.