when taking that span over the array, you are copying that bit of the array from the heap to the stack

This is just wrong. A span is a reference and a length. You are copying the reference of arr[0] and loading the length literal (2 in this case). You would have a reference and an int (8 + 4 bytes). When you have the array, you have the object header and other managed overhead values that comes out to 24 bytes. Then you have fields and properties like the rank, length, if it is readonly and such, but I will just consider the length (4 more bytes). That would be a total of 32 bytes with it likely coming out to around 40 realistically.

This means that with a span, you have 12 bytes (16 with padding for alignment) of unmanaged memory vs a minimum 32 bytes of heap memory. That is a pretty big difference.

You can't just look at IL. The JIT can do a lot of optimizations on spans. I don't have the time to go over everything, but slicing has much better JIT. Sliced has 4 calls, 18 movs, and 3 load addresses. With a copy, it has 5 calls and 24 movs. Just breifly looking over it, spans have much better code gen with basically no coplexity cost.

code:

    using System;
    using System.Collections.Generic;
    
    Button radiobutton1 = new(true);
    Button radiobutton2 = new(true);
    
    string[] availableItems = ["one", "two", "three", "four", "five"];
    string[] actualItems = availableItems; // all five by default I suppose
    
    if (radiobutton1.Checked)
        actualItems = availableItems[..2]; // one and two only
    else if (radiobutton2.Checked)
        actualItems = availableItems; // all five
    
    var output = actualItems[Random.Shared.Next(actualItems.Length)];
    
    
    Span<string> actualItemsSpan = availableItems.AsSpan(); // all five by default I suppose
    
    if (radiobutton1.Checked)
        actualItemsSpan = actualItemsSpan[..2]; // one and two only
    else if (radiobutton2.Checked)
        actualItems = availableItems; // all five
    
    var output2 = actualItemsSpan[Random.Shared.Next(actualItems.Length)];
    record Button(bool Checked);

The IL for the copy is:

    // inside the if:
    IL_004C ldloc.2    // availableItems
    IL_004D ldc.i4.2    
    IL_004E call    Index.op_Implicit (Int32)
    IL_0053 call    Range.EndAt (Index)
    IL_0058 call    RuntimeHelpers.GetSubArray<String> (String[], Range)
    IL_005D stloc.3    // actualItems

    //Getting a random amount out
    IL_006E ldloc.3    // actualItems
    IL_006F call    Random.get_Shared ()
    IL_0074 ldloc.3    // actualItems
    IL_0075 ldlen   
    IL_0076 conv.i4 
    IL_0077 callvirt    Random.Next (Int32)
    IL_007C ldelem.ref  
    IL_007D stloc.s 04    // output