Fluorination (155kJ/mol) seems to have relatively high activation energy. The initial reaction (chain initiation) - that is, the homolytic cleavage of a halogen molecule - must, however, occur only a few times.
The subsequent reactions (chain propagation) between a halogen radical and methane, and then between a methyl radical and a halogen molecule, yield another halogen radical.
Therefore, one start reaction may initiate thousands of fluorination reactions. In addition, fluorination is very exothermic, the reaction enthalpy is −431kJ/mol. As a result, the reaction itself provides enough energy for additional initiation reactions. Hence, the halogenation of alkanes is violent and very fast in Fluorine.