Unknown set types go through a lot of scanning - often involving reflection - before a simple copy constructor is used. These include the commonly used:

ConcurrentHashMap$KeySet
ConcurrentHashMap$EntrySet
FastCopyHashMap$KeySet
FastCopyHashMap$EntrySet
Collections$EmptySet
Collections$SingletonSet
Collections$EmptyList
Collections$SingletonList
Collections$EmptyMap
Collections$SingletonMap

these should be treated accordingly to avoid reflection calls.