Hi, no app will help you with that - you need to find the root cause of the lag in order to resolve it.
1) Try deleting old apps you no longer use, or apps that are power hungry
2) Try deleting media content on the phone, providing you have backed up the data
3) When do you experience this slow speed, is it constant or when you are performing specific tasks?
4) It might be worth while considering a factory reset to freshen it up a little.
Come back to us if the above don't help and we can take it from there