Why is List.Sort using Comparer.Default more than twice as fast as an equivalent custom comparer?

Question

Results

Using a list of 10 million random ints (same seed each time, average of 10 repetitions):

listCopy.Sort(Comparer.Defau

Answer 1

ILSpy decompiles thus:

    public override int Compare(T x, T y)
    {
        if (x != null)
        {
            if (y != null)
            {
                return x.CompareTo(y);
            }
            return 1;
        }
        else
        {
            if (y != null)
            {
                return -1;
            }
            return 0;
        }
    }

The null checks will always evaluate as true for a value type, so they will be optimized away; the end result will be

public override int Compare(T x, T y)
{
    return x.CompareTo(y);
}