Turns out using lambdas to control pins is lovely and correct, but terribly slow. Back to a more traditional method.

EightBit/Register16.cs

@@ -46,7 +46,6 @@ namespace EightBit
 
         public Register16(Register16 rhs)
         {
-            ArgumentNullException.ThrowIfNull(rhs);
             this.Low = rhs.Low;
             this.High = rhs.High;
         }
@@ -68,7 +67,6 @@ namespace EightBit
 
         public static bool operator ==(Register16 left, Register16 right)
         {
-            ArgumentNullException.ThrowIfNull(left);
             return left.Equals(right);
         }
 
@@ -88,7 +86,6 @@ namespace EightBit
 
         public void Assign(Register16 from)
         {
-            ArgumentNullException.ThrowIfNull(from);
             this.Assign(from._low, from._high);
         }
     }