factory reset will reduce the lag and the not starting apps and it won't remove the root
formatting system and installing a stock ROM through Odin will certainly solve the problem
it's not right that you should unroot before installing stock ROM you can do it without doing anything before
and there is no any deference between installing USA ROM or the French one
if you decide what you wanna do just tell us what do you want and we'll guide you
What is the easiest method to install the USA rom... where can I find a guide for it?