I updated to ICS a few days ago. I have been experiencing lag, as well, particularly after closing apps and returning to the home screen. I also been hearing a looping melody that goes off for no apparent reason. It woke up in the morning. No, I don't have an alarm set up. I don't know how to turn it off. I had to put the phone on vibrate when it goes off. I checked my settings, and I don't know where it is coming from. I spoke to customer service and they mentioned I should do a hard reset. I did a hard reset before I upgraded to ICS. I only installed news and social apps. The phone is not rooted and is the legitimate Samsung T-Mobile ICS firmware.
I just checked my notifications and it mentioned that my prepaid T-mobile SIM is not compatible with Wi-Fi calling. I dismissed the notification and it finally stopped, for now.