BigInteger.ToString is slow

using System;
using System.Diagnostics;
using System.Numerics;

namespace Tests
{
    class Program
    {
        private static BigInteger silnia(int i)
        {
            BigInteger result = 1;
            for (int x = 1; x <= i; x++)
            {
                result *= x;
            }
            return result;
        }

        public static void Main(string[] args)
        {
            Console.Write("Wprowadź liczbę, której silnię pragniesz policzyć: ");
            int n = int.Parse(Console.ReadLine());
            Stopwatch watch = new Stopwatch();

            watch.Start();
            BigInteger resultF = silnia(n);
            TimeSpan factTime = watch.Elapsed;

            watch.Restart();
            string resultAsString = resultF.ToString();
            TimeSpan convTime = watch.Elapsed;

            Console.WriteLine("Wynik: {0}", resultAsString);
            Console.WriteLine("Czas obliczeń: {0} sekund", factTime.TotalSeconds);
            Console.WriteLine("Czas konwersji: {0} sekund", convTime.TotalSeconds);
        }
    }
}