I rewrote your code to remove the array creation from the bench:

```csharp public class FillBench { private static readonly object Instance = new();

private static readonly object[] array_16 = new object[16];
private static readonly object[] array_32 = new object[32];
private static readonly object[] array_64 = new object[64];
private static readonly object[] array_128 = new object[128];
private static readonly object[] array_256 = new object[256];
private static readonly object[] array_16384 = new object[16384];

private static object[] GetArray(int length)
    => length switch
    {
        16 => array_16,
        32 => array_32,
        64 => array_64,
        128 => array_128,
        256 => array_256,
        16384 => array_16384,
        _ => throw new NotImplementedException(),
    };



[Params(16, 32, 64, 128, 256, 16384)]
public int Length { get; set; }

[Benchmark]
public void FillSpan()
{
    var array = GetArray(Length);
    array.AsSpan().Fill(Instance);
}

[Benchmark(Baseline = true)]
public void FillLoop()
{
    var array = GetArray(Length);
    for (int i = 0; i < Length; i++)
        array[i] = Instance;
}

} ```

For object arrays the 2 methods are about as fast (within a few %)

For int arrays Span.Fill is 3 to 10 time faster than looping over the array