First of all,
Are you on Android 4.3 or 4.1.2?
If you are on 4.3, it is expected to be a little laggy or buggy (No offence intended)
For this, what you could do is wait for a new update or downgrade to 4.1.2.
There is also another fix to make your phone a little less laggy.
For that you need to root your phone and edit the build.prop.
Second of all, you can try CyanogenMod without voiding your warranty.
For that you have to backup your TA and then unlock the bootloader. That is how you can use CyanogenMod and then again restore your TA when you are going to a Sony Centre or something like that.
A factory reset may solve somethings but not all.