Nullable<T> のコスト高すぎると思うの

BitFlags.cs

using System.Runtime.CompilerServices;

#if false
M(new A { X = 1 });
M(new B { X = 1 });
M(new A { Y = 2 });
M(new B { Y = 2 });
M(new A { X = 1, Y = 2 });
M(new B { X = 1, Y = 2 });
M(new A { });
M(new B { });

static void M<T>(T x) where T : IA
{
    Console.WriteLine($"{x.X} {x.Y}");
}
#endif

Console.WriteLine(Unsafe.SizeOf<A>()); // 40
Console.WriteLine(Unsafe.SizeOf<B>()); // 16

interface IA
{
    byte? W { get; set; }
    int? X { get; set; }
    long? Y { get; set; }
    short? Z { get; set; }
}


struct A : IA
{
    public byte? W { get; set; }
    public int? X { get; set; }
    public long? Y{ get; set; }
    public short? Z { get; set; }
}

struct B : IA
{
    private byte _flags;
    private byte _w;
    private short _z;
    private int _x;
    private long _y;

    public int? X
    {
        get => (_flags & 1) != 0 ? _x : null;
        set
        {
            if (value.HasValue)
            {
                _flags |= 1;
                _x = value.Value;
            }
            else
            {
                _flags &= 0xFE;
            }
        }
    }

    public long? Y
    {
        get => (_flags & 2) != 0 ? _y : null;
        set
        {
            if (value.HasValue)
            {
                _flags |= 2;
                _y = value.Value;
            }
            else
            {
                _flags &= 0xFD;
            }
        }
    }

    public short? Z
    {
        get => (_flags & 4) != 0 ? _z : null;
        set
        {
            if (value.HasValue)
            {
                _flags |= 4;
                _z = value.Value;
            }
            else
            {
                _flags &= 0xFB;
            }
        }
    }

    public byte? W
    {
        get => (_flags & 8) != 0 ? _w : null;
        set
        {
            if (value.HasValue)
            {
                _flags |= 8;
                _w = value.Value;
            }
            else
            {
                _flags &= 0xF7;
            }
        }
    }
}