This is a great question, but the answer depends on how you are taking our classes:
With the Start ASL Online Course (http://online.startasl.com), you can slow the videos by clicking on the clock button that appears while hovering your cursor over the videos.
With the Offline Course (http://online.startasl.com) or Teacher's Curriculum (http://teachers.startasl.com), you can only slow the videos if you are using a video player on your computer that supports that ability. Two video players that offer this ability are Windows Media Player 12 (by right-clicking in the player and adjusting Enhancements > Play Speed Settings) and Quicktime Player (by pausing the video and using your arrow keys).
With the Free ASL 1 Course (https://www.startasl.com/subscribe), the videos are hosted on Youtube, so you can use the Youtube controls to slow down the videos.