They are immutable by default, but it's expensive to get a new data structure with some changes in it. That's what persistent data structures for. I personally like this explanation - https://github.com/vacuumlabs/persistent#what-are-persistent...
Your classes are 10-20x slower in every other way, which definitely would offset any imaginary expense you think calling `mutableCopy` costs.
I have to tell you in 10+ years of writing Objective-C where I was like [myArray mutableCopy] is super slow, I need a replacement. I've wished for an NSOrderedDictionary though.
Then,
// yourDictionary is NSMutableDictionary, [yourDictionary count] > 20
NSMutableDictionary *mutable = [yourDictionary mutableCopy];
mutable[@"old_key"] = @"new_value";
NSDictionary *changedDict = [NSDictionary dictionaryWithDictionary:mutable];
// yourDictionary is AAPersistentHashMap, [yourDictionary count] > 20
AAPersistentHashMap *changedDict = [yourDictionary setObject:@"new_value" forKey:@"old_key"];