account the difference in the crystals formed through rapid and slow crystallization.
Slow crystal growth consists of a tiny seed crystal forming and other molecules of the crystal joining the crystal lattice bit by bit. The end result may not be a group of tiny crystals but the initial seed crystal can become quite large. Slow crystal growth gives compounds that are purer than crystals formed from rapid growth.
Crystals that form rapidly (for example when a precipitate forms from the addition of one material to another that gives an insoluble compound) are not as pure because they tend to trap other ions/molecules within the crystal lattice. This goes by a number of different names; the general term is co-precipitation (although the use of that term is not as popular as it was 50 years ago) or occlusion. The so-called guest molecule, often a solvent molecule, gets caught in the cage-like structure of the crystal lattice. Often these are called clathrates (from the Greek cage-like). Rapidly formed crystals, because they are not as pure, must be purified if possible, either by re-crystallization or by digestion. In the case of organic compounds the use of recrystallization is preferred. In inorganic chemistry (for example the pptn of BaSO4) recrystallization isn't possible due to the insolubility of BaSO4; therefore, digestion is used for those materials.
P. S. If the ion being trapped fits into the crystal lattice, the result is mixed crystal formation; if it does not fit into the crystal lattice it is called a clathrate.