When you turn your A/C on, cars tend to be abit laggier because the A/C gets power from the motor to run it. As for your clutch, if it does slip alot, it does mean its on its way but if you drive the car like a baby all the time, it will increase the life of your clutch. If you are interested in changing it, it would roughly be around the $600 mark including new clutch and the labour to do it. If I were you, I wouldnt be so worried. (Unless its really that bad).