How is the overall performance of iphone 4s with iOS 8.3?

How is batttery life on average phone use?
How is app and gaming performance?
Any other problems?

If the storage is more than 16gb then there no such issue with ios8.3 in 4s… Certainly the battery backup improves in ios 8.3

app and gaming performance is very good but sometimes game stop and game shut down its own

else all good

4S is quite old now but thats a great phone, I would stick with 8x though and not upgrade to 9 as the hardware may not be able to handle it well.

I think there wouldn't be any problem as iPhone 4S is a good device and you may also install Leo Privacy Guard in case you need a good security app, it's the best one when it comes to security.