IdentityHashMap class has one tuning parameter for performance improvement: expectedMaxSize.

This parameter is the maximum number of key-value mappings that the map is expected to hold.

We can use this parameter is used to determine the number of buckets initially in the hash table. The precise relationship between the expected maximum size and the number of buckets is unspecified.

If the number of key-value mappings exceeds the expected maximum size, the number of buckets is increased.

Increasing the number of buckets is also known as rehashing. Rehashing may be fairly expensive. So it is better to create identity hash maps with a sufficiently large expected maximum size.

But iteration over a Map collection requires time proportional to the number of buckets in the hash table. So iteration may take extra time due to large number of buckets.

Therefore the value of expectedMaxSize should be set in consideration with both of these aspects.