One thought that I think has worked for me in the past. Double-click the home button to get to the fast-app switching. Hold down an icon, wait for the shake, and hit that X on every app currently 'open.' Then restart the device.
I don't think those 'open' apps are supposed to be taking any memory or anything while in that state, but I don't believe it. Even restarting the phone doesn't clear those.
That's the only thing I can suggest. I have had that lag before, and it is highly annoying.