OK, so you've definitely upgraded as far as processor goes. More than likely it's just the phone settling in after transferring all your apps and stuff from your old phone. It could also be an app that's not running correctly because of the switch. If it doesn't get any better after a day or so you might have to manually install your apps.
Sometimes app data isn't compatible from phone to phone and can potentially cause the app to use too much RAM thus slowing the phone down. So you'll need to do a factory reset and start from scratch, unfortunately.
If that doesn't fix it you could potentially have a defective phone.