Sorry to post again !!
Okay, it started to lag as described by others. So I went back to 4.0.4... The battery life is better on Jelly Bean because it can reduce the screen's power (not just brightness)
I did this today, my last attempt with JB...
In Dial :: *2767*3855# (it formats everything, even sdcard, so backup your things before doing this !!!)
I configured ICS from the welcome screen (no gmail configuration, etc.)
Installed JB 4.1.2 from Kies.
I disabled the programs mentionned at my previous post...
There was still a lag when unlocking the phone. Tried Holo Locker and didn't do anything... still lags.
Then, I searched on the net :
It seems TouchWiz has problems rendering Widgets on homescreen, so, I installed Nova Launcher to replace it.
For now, when receiving a SMS, after unlocking, the wallpaper can become black, but there is no lag like before.