The new tiles only appears on the opposite of your move direction. Your code does not seem to care about that. Maybe it will perform better if this is considered?
Couple months ago, I met Andrew Ng at a meet up and asked him why there's deadline for the courses (comparing to Udacity model). He said they did a study, courses with deadlines tends to have more students.
I found that it's very plausible. I still have couple of Udacity classes half finished, thinking I can always come back later, while working furiously on Coursera's deadlines...
Born and raised in China, I can testify that working ethic was bad before they adopted market economics. What skylan_q described was happening in China as well.