If your phone isn't in factory condition try it in safe mode
- Turn off the device.
- Press and hold down the power key until the phone vibrates.
- Wait until the Xperia™ logotype disappears.
- Press and hold the volume down key. The device then restarts in safe mode.
- To exit safe mode, restart the device.
(or you can do a factory reset to test phone in stock condition, but you will lose your datas)
If no issue in safe mode probably an app is creating a problem
You can also try to do firmware updates that could improve system performance, if not already done
Anyway remember that your phone has only 512 MB of RAM and for this reason can't handle a lot of background apps.
