Tried but no luck. Windows phone 8.1 messed up my Lumia 520 °raded its performance too.Well, ok.
Assuming you want help, try a soft reset first. (hold power + volume down until vibrate) If that doesn't help, a hard reset (erases your phone) will fix a lot more. (settings > about > reset my phone)
However make sure stuff is backed up.
Tried but no luck. Windows phone 8.1 messed up my Lumia 520 ?raded its performance too.