Small setup changes can make a big difference to how a kart feels.
Start with tyre pressures, because they are quick to adjust and have a major effect on grip. Front and rear track width can change how the kart turns and releases through corners. Ride height, axle stiffness and seat position can also make a big difference.
The golden rule is to change one thing at a time. If you change too much at once, it becomes almost impossible to know what improved or worsened the kart.