The way that I believe most people have their Titan's configured is such that they are slower than the other classes. I also typically play as a Defender, in a support role, so I'm trying to keep people alive by throwing up bubbles when needed and sniping heavier enemies from afar, which enables the speeder to move that much further ahead and I get left behind at times. I get that Titans can be configured to keep up with the pack, but I don't think it's typical.