Optimizing Karatsuba Implementation

So, I'm trying to improve some of the operations that .net 4's BigInteger class provide since the operations appear to be quadratic. I've made a rough Karatsuba implementation but it's still slower than I'd expect.

The main problem seems to be that BigInteger provides no simple way to count the number of bits and, so, I have to use BigInteger.Log(..., 2). According to Visual Studio, about 80-90% of the time is spent calculating logarithms.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Numerics;

namespace Test
{
    class Program
    {
        static BigInteger Karatsuba(BigInteger x, BigInteger y)
        {
            int n = (int)Math.Max(BigInteger.Log(x, 2), BigInteger.Log(y, 2));
            if (n <= 10000) return x * y;

            n = ((n+1) / 2);

            BigInteger b = x >> n;
            BigInteger a = x - (b << n);
            BigInteger d = y >> n;
            BigInteger c = y - (d << n);

            BigInteger ac = Karatsuba(a, c);
            BigInteger bd = Karatsuba(b, d);
            BigInteger abcd = Karatsuba(a+b, c+d);

            return ac + ((abcd - ac - bd) << n) + (bd << (2 * n));
        }

        static void Main(string[] args)
        {
            BigInteger x = BigInteger.One << 500000 - 1;
            BigInteger y = BigInteger.One << 600000 + 1;
            BigInteger z = 0, q;

            Console.WriteLine("Working...");
            DateTime t;

            // Test standard multiplication
            t = DateTime.Now;
            z = x * y;
            Console.WriteLine(DateTime.Now - t);

            // Test Karatsuba multiplication
            t = DateTime.Now;
            q = Karatsuba(x, y);
            Console.WriteLine(DateTime.Now - t);

            // Check they're equal
            Console.WriteLine(z == q);

            Console.Read();
        }
    }
}

So,