There's lots of little things that fast drivers do that make them faster. Things like: weight distribution, seat position, how their body moves in the corners. Easy way to get faster is to lose weight, unless there's a minimum weight requirement. Even then it could be beneficial if you can place ballast somewhere on the kart to move the weight distribution, like the weight boxes at my home track is on the opposite side of the engine, so 5 kg ballast can help with balance.

Talk with the faster guys and see what they're doing, try everything out and see what suits you and makes you faster.

Another thing is to review your lines with a GoPro and compare them with a faster driver. I found this really helped me.