Programming

I'm gonna hazard a guess, just cause I'm curious, that you're coming from JavaScript.

Regardless, the answer's basically the same across all similar languages where this question makes sense. That is, languages that are largely, if not completely, object-oriented, where memory is managed for you.

Bottom line, object allocation is VERY expensive. Generally, objects are allocated on a heap, so the allocation process itself, in its most basic form, involves walking some portion of a linked list to find an available heap block, updating a header or other info block to track that the block is now in use, maybe sub-dividing the block to avoid wasting space, any making any updates that might be necessary to nodes of the linked list that we traversed.

THEN, we have to run similar operations later for de-allocation. And if we're talking about a memory-managed language, well, that means running a garbage collector algorithm, periodically, that needs to somehow inspect blocks that are in use to see if they're still in use, or can be automatically de-allocated. The most common garbage-collector I know of involves tagging all references within other objects, so that the GC can start at the "root" objects and walk the entire tree of references within references, in order to find any that are orphaned, and identify them as collectable.

My bread and butter is C#, so let's look at an actual example.

public class MyMutableObject
{
    public required ulong Id { get; set; }

    public required string Name { get; set; }
}

public record MyImmutableObject
{
    public required ulong Id { get; init; }

    public required string Name { get; init; }
}

_immutableInstance = new()
{
    Id      = 1,
    Name    = "First"
};

_mutableInstance = new()
{
    Id      = 1,
    Name    = "First"
};

[Benchmark(Baseline = true)]
public MyMutableObject MutableEdit()
{
    _mutableInstance.Name = "Second";

    return _mutableInstance;
}

[Benchmark]
public MyImmutableObject ImmutableEdit()
    => _immutableInstance with
    {
        Name = "Second"
    };

Even for the most basic edit operation, immutable copying is slower by more than 7 times, and (obviously) allocates more memory, which translates to more cost to be spent on garbage collection later.

Let's scale it up to a slightly-more realistic immutable data structure.

public class MyMutableParentObject
{
    public required ulong Id { get; set; }

    public required string Name { get; set; }

    public required MyMutableChildObject Child { get; set; }
}

public class MyMutableChildObject
{
    public required ulong Id { get; set; }

    public required string Name { get; set; }

    public required MyMutableGrandchildObject FirstGrandchild { get; set; }
            
    public required MyMutableGrandchildObject SecondGrandchild { get; set; }
            
    public required MyMutableGrandchildObject ThirdGrandchild { get; set; }
}

public class MyMutableGrandchildObject
{
    public required ulong Id { get; set; }

    public required string Name { get; set; }
}

public record MyImmutableParentObject
{
    public required ulong Id { get; set; }

    public required string Name { get; set; }

    public required MyImmutableChildObject Child { get; set; }
}

public record MyImmutableChildObject
{
    public required ulong Id { get; set; }

    public required string Name { get; set; }

    public required MyImmutableGrandchildObject FirstGrandchild { get; set; }
            
    public required MyImmutableGrandchildObject SecondGrandchild { get; set; }
            
    public required MyImmutableGrandchildObject ThirdGrandchild { get; set; }
}

public record MyImmutableGrandchildObject
{
    public required ulong Id { get; set; }

    public required string Name { get; set; }
}

_immutableTree = new()
{
    Id      = 1,
    Name    = "Parent",
    Child   = new()
    {
        Id                  = 2,
        Name                = "Child",
        FirstGrandchild     = new()
        {
            Id      = 3,
            Name    = "First Grandchild"
        },
        SecondGrandchild    = new()
        {
            Id      = 4,
            Name    = "Second Grandchild"
        },
        ThirdGrandchild     = new()
        {
            Id      = 5,
            Name    = "Third Grandchild"
        },
    }
};

_mutableTree = new()
{
    Id      = 1,
    Name    = "Parent",
    Child   = new()
    {
        Id                  = 2,
        Name                = "Child",
        FirstGrandchild     = new()
        {
            Id      = 3,
            Name    = "First Grandchild"
        },
        SecondGrandchild    = new()
        {
            Id      = 4,
            Name    = "Second Grandchild"
        },
        ThirdGrandchild     = new()
        {
            Id      = 5,
            Name    = "Third Grandchild"
        },
    }
};

[Benchmark(Baseline = true)]
public MyMutableParentObject MutableEdit()
{
    _mutableTree.Child.SecondGrandchild.Name = "Second Grandchild Edited";

    return _mutableTree;
}

[Benchmark]
public MyImmutableParentObject ImmutableEdit()
    => _immutableTree with
    {
        Child = _immutableTree.Child with
        {
            SecondGrandchild = _immutableTree.Child.SecondGrandchild with
            {
                Name = "Second Grandchild Edited"
            }
        }
    };

Not only is performance worse, but it drops off exponentially, as you scale out the size of your immutable structures.

Now, all this being said, I myself use the immutable object pattern FREQUENTLY, in both C# and JavaScript. There's a lot of problems you encounter in business logic that it solves really well, and it's basically the ideal type of data structure for use in reactive programming, which is extremely effective for building GUIs. In other words, I use immutable objects a ton when I'm building out the business layer of a UI, where data is king. If I were writing code within any of the frameworks I use to BUILD those UIs (.NET, WPF, ReactiveExtensions) you can bet I'd be using immutable objects way more sparingly.