There are three people in my group who use Camtasia, and we all experience the exact same issue. It doesn't start out that way, but once you begin adding more and more video clips, voice-overs, animations, etc., it progressively gets worse. Saving and closing, then re-opening the Camtasia file sometimes helps (a little). I'm in the process of getting a much more powerful machine that will hopefully help resolve this issue. I'm also in the process of transitioning to Premier Pro, because this software doesn't seem to have the same "lag" issue.