Moving fast through the water requires a couple things: a great catch and a high-speed turnover. Sometimes, the harder you try, the less return you get on the water.
As you lower your head into the water, you'll feel the release of resistance. At this point, it's important to keep your focus on the front part of your stroke. Continue kicking hard so your velocity stays up, and remember to "let go" of the water soon enough in the back, or you may end up losing your paddles.