There is a program called MTN or MovieThumbnailer that can create a contact sheet of any size video in less than one-second. It uses FFmpeg as the driving engine. You can drag&drop a single video, several videos, or an entire folder on the exe to create any number of thumbnail contact sheets. A better way to use it is to drag&drop onto a batch file that sets your own custom parameters for the sheets, such as the number of thumbnails per page, columns, rows, sheet size, etc. I assume it supports commandline usage, as well.
SourceForge seems to be down at the moment, but when it returns...
movie thumbnailer (mtn)
http://moviethumbnail.sourceforge.nethttp://www.sourceforge.net/projects/moviethumbnail/I'll post the batch file if needed.