It is not certain that the slow metabolism of reptiles is primitive and that birds and mammals have developed homeothermy independently.
Because there are certain similarities between the homeothermy of birds and that of mammals that seem less likely to be coincidences, there exists an alternative hypothesis, that homeothermy and fast metabolism has already evolved in the ancestor of all amniotes.
Later, in the ancestors of crocodyles, of turtles and in that of lizards and snakes (both etymologically and cladistically, it would be better to apply the term "reptiles" only to lizards and snakes), poikilothermy and slow metabolism have evolved as an adaptation that allows the reptiles to live using much less food than needed by birds and mammals.
This reversal appears especially likely for crocodiles, whose ancestors were terrestrial and agile.
Such a reversal is also consistent with the fact that reptiles need a high body temperature for a normal activity, unlike the amphibians, for which there is no doubt that they did not have an ancestor with homeothermy. To reach their required body temperature, reptiles need to use external sources, e.g. basking in sun light. Because of their dependence on external sources of heat, reptiles become rare or completely absent in colder climates, where amphibians are still abundant.