Try deleting your system cache. This doesn't lose any data (or shouldn't) or settings. Google how to stating your phone type.
My Samsung involves switching off, then switching in again while pressing home and volume up (or is it down?). Then it boots into a mode where you have options, one if which is 'clear system cache
Phone inevitably runs better if you do this every now and then.
If its still poor, I've done S factory reset once or twice. Make sure you back up whatever stuff you want first though, as it wipes everything.
Good thing is, once you set it up afresh it will automatically reload all your apps from google.