As the other person said, your probably should do a factory reset. You may even want to select the option to clear the internal sd card when doing it.
Basically depending on how mucked up the software was before upgrading, it only makes it more pronounced after the upgrade. A factory reset will remove all the apps and make you reload them, setting them up cleanly for 4.1.2.
Don't forget we are also doing more with the same resources. It is just going to be slower at times. Mine is pretty responsive most of the time, but it has it's moments of lag. Mostly when jumping between apps allot.