The answer is; TRUE
When the saturated solution is allowed to cool slowly, the molecules in the solution have the time to displace impurities that try to attach to the growing lattice because they fit perfectly, geometrically then the impurities. This way the lattice grows larger.
If the solution is cooled fast, the impurities have little time to be displaced from the lattice hence the geometry of the crystal lattice is disturbed and the crystals are smaller and less pure.