We have a few reasons for signing faster in the videos. One reason is because it doesn't really matter at that point. We don't expect you to be able to follow everything that we're saying. That's when we like you to focus on what you're looking for in what we're signing--whether it be a number or a fingerspelled word, etc. Because even when you're a very skilled signer, you won't be able to pick up everything someone is signing to you. Most of the time, you understand what they're saying based on context, handshapes, movements, etc. and not on each individual sign. We try to teach that a bit early because often, when signers finally figure that out, they are so used to understanding each individual sign that it is difficult to learn how to decipher the signs they don't know.
And, signing faster gives some students the ability to learn advanced signing early on if they want to. A lot of students watch the videos over and over and over until they understand it, and it gets easier and easier as they move on. We think that's a huge advantage compared to in-person classes because students really can learn at their own pace. But, of course, like I said, understanding everything in the videos is not absolutely necessary for learning ASL. The signing speed also helps people who have signed before to brush up on their signing a bit faster.
So, all in all, it's not absolutely necessary for you to understand everything in the videos, but being able to watch them over and over does give you the ability to do so if you want to. :) One visitor actually had some insight on this as well. You can read her article here: Learning Beyond the Sign
If you're interested in watching the videos in slower motion, you should look into signing up for our official Online Course. The videos in the Online Course use a video player that allows you to watch the videos in slower or faster motion. This can help immensely if you're trying to learn advanced signing early on or if you're really having difficulty catching all the signs in the free videos.