If you have bought your Nexus S smartphone outside of any operator pack, you are getting regular Other The Air updates straight from Google.
For example, when I bought mine 9 months back, it was running Gingerbread 2.3.6. Then I got OTA updates for :
Ice Cream Sandwich 4.0.4 : After that update, my phone was showing some strange behaviour mostly on the desktop screen.
Sometimes some parts of the screen where not refreshed, some fonts where becoming bold, ...
Jelly Bean 4.1.1 : After that update, my phone became extremely slow.
For example, I was sometimes left 5 seconds on a white screen while entering the SMS application.
Same type of behaviour entering Gmail, ...
These problems come most of the time from the phone's system caches that might have been corrupted by the OTA update.
The clue is to purge these cache. After this operation, you will rediscover your Nexus S in full speed. Jelly Bean will be blazing fast.
This operations needs your phone to be rooted. If your phone is already rooted you won't loose any data, it's totally transparent.
But if your phone is not rooted, you'll need to root it first and it will wipe all your data.
1. Pre-requisite : Rooted phone
If your Nexus S is already rooted, just skip this step.
If not, you need to root it. A lot of guides are available on Internet.
You can follow Nexus S - Root your phone under Ubuntu Linux.
2. Wipe the cache
Restart you phone and boot it into Recovery mode (Power + Volume Up).
Once in FastBoot screen, select Recovery and press Power
You now enter Recovery.
Here comes the job. You now need to :
- Wipe the Cache
- Wipe the Dalvick cache
Depending on the recovery program you installed, the menus may look different.
If you installed TWRP Recovery (following Nexus S - Root your ICS or JB phone under Ubuntu Linux), go to the menu wipe.
You need to select both cache and Dalvik cache.
Operation should be really fast.
At the end, when both cache are wiped, go back in the menu and select Reboot.
3. First Reboot
During first reboot, your phone will need to reconstruct its Dalvik cache.
This will take some time, usually between 1 and 2 minutes.
A progression screen will keep you informed.
Once done, your phone will finish its boot process.
You should not get a brand new experience with a lightning speed Jelly Bean Nexus S.
Hope it helps !