Back when I used to run, the first two laps were always the fastest, typically the 2nd on Bridgestone YHCs. Just like any racecar, lap times will drop off as the tires heat up etc. Half a second is a bit large, but if you are using older tires that is going to happen. If you are feathering the inside of the tires that means the front is pushing or the kart is bound up overall. Try changing some of the chassis bars or rear width, dont remember what way to go on caster etc on a kart to help that problem anymore.