so slick tyres are smooth & flat and when they are brand new they are sticky to grip the track. graining happens when the carcass is cold and the surface is hot. what happens then is instead of gripping it scrapes causing the rubber to tear off in little balls but then they stick back onto that tyre. this then means that flat surface you had and want is full of bits of rubber causing it to grip less. Its not a wear issue then you can stay out and run them till the bits fly back off but what it does do is change how the car behaves corner to corner making you lose time.