What happened to my iPhone 5?

I bought a 32GB iPhone 5 on 6 Feb and it was running efficiently until 16 Feb when I entered the lock screen slower than usual. I clicked the 'power' button on top and it took a second for the screen to appear before I could 'slide to unlock' while the rest of my friends' were instant despite containing less space and more apps than mine. I didn't overcharge the phone and took great care of it. Heck, I treated it like a jewellery instead of a smart phone. Deleting all content and resetting the phone were to no avail. I don't have this problem when charging the phone but as soon a the battery capacity's full and the pin unplugged, this problem raises its ugly head and persisted until now. Please help!!!

I'm a first-time iPhone user.