To be honest mine was a bit sluggish the first time I turned it on. Have you tried powering the phone off then back on again?
Sometimes (rarely) when I get lag on an app it's worth closing it down from the iPhone version of task manager, just double tap the middle home button and it'll bring up a list of currently open apps at the bottom of your screen. if you hold down on one for a second a red icon will appear and you can shut them down in the background by clicking.
Failing that it might be worth turning mobile data off then back on again. Go to Settings > General > Network and slide from on to off, then repeat under 'Mobile Data'
Finally, under the same 'Network' menu (as above) go to 'Mobile Data Network'. What settings do you have? I have the following successfully working on a pay monthly SIM: