If you have already passed 15 hours of playtime on your attempt to get the Speedstar Trophy, their is another way to get it. When your in-game clock gets to 99:59:59 it will reset back to 0:00:00 and change colors from Blue to Green. This is supposed to let you know that you have played for over 100 hours. However, the Trophy will read the actual game time, which will show as below 15 hours. All you have to do is defeat the final boss before it passes the 15 hour mark again, and the trophy will unlock.