I haven't got to playing this one myself, My system needs to be updated before I can. So I've been watching more let's plays then I care to admit. Watching I felt like they put the same pressure on finding your son as they did for finding your dad in FO3. When you think about it they are giving you the same threat vs freedom that Bethesda is famous for. They don't force you to complete the game in a set amount of time like the first Fallout with its deadline. While they are agiving you that freedom, its more how much time do you wish to spend on the main quest vs the side quest. By the way, I looked up Fallout 4 on Leaderboard. The current speed runners listed having completing the main quest between one and two hours. That could give you inspiration for getting your tail in gear and finishing the main quest.