My game really lagged/slowed down at the very beginning, but it was a brief moment and hasn't done it since. I believe another poster mentioned this happening.
Only grievance I have thus far. I did all the patrol missions and just finished the 1st traffic cop mission. It took me roughly an hour to do, because I kept getting interrupted to do all the lil side missions. I did 8 of those, and while those aren't really that fun... its a good break from the cases.