The mechanism of "caching the last closure created for a prototype to try to reuse it the next time a closure for that prototype is created" was removed. There are several reasons: - It is hard to find a natural example where this cache has a measurable impact on performance. - Programmers already perceive closure creation as something slow, so they tend to avoid it inside hot paths. (Any case where the cache could reuse a closure can be rewritten predefining the closure in some variable and using that variable.) - The implementation was somewhat complex, due to a bad interaction with the generational collector. (Typically, new closures are new, while prototypes are old. So, the cache breaks the invariant that old objects should not point to new ones.)