Samsung Galaxy SIII sluggish after upgrading to Jellybean 4.1

[Chaitanya Agrawal]

Hi,
I upgraded my Samsung Galaxy SIII to Android Jellybean 4.1
The phone has been very sluggish after that, to the extent, it takes a full 10 seconds for the keypad to open.
On getting incoming calls, the caller ID won't recognise the number for couple of seconds.
I have tried rebooting and clearing the RAM many times, but it doesn't help.
I have plenty of free space. Have used just 1.83 GB of the available 11.35 GB internal storage.
Sometimes, even the clock widget gets stuck and doesn't show the correct time (screenshot attached).

Help please!!

 

[sukant08]

first thing first. A factory reset after the JB update. I also had general sluggishness - but not to the extent of, things taking 10 secs to open - and after a factory reset it went away.

 

[Chaitanya Agrawal]

What exactly does a factory reset do? Will the OS go back to 4.0?
And will I loose my settings and data?