Matching a generated string of random letters to an input

I made a program in Python and wanted it to be faster, so I wrote it on C# because it's compiled. To my surprise, the Python program is much faster. I guess there is something wrong with my C# code, but it is pretty simple and straightforward, so I don't know. They are structured about the same way.

C#:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Diagnostics;

//This program generates a string of random lowercase letters and matches it to the user's input
//It does this until it gets a match
//It also displays the closest guess so far and the time it took to guess

namespace Monkey
{
    class Program
    {
        static string userinput()
        {
            //Takes user input, makes sure it is all lowercase letters, returns string
            string input;

            while(true)
            {
                input = Console.ReadLine();

                if (Regex.IsMatch(input, @"^[a-z]+$"))
                {
                    return input;
                }
            }
        }

        static string generate(int len)
        {
            //generates string of random letters, returns the random string
            Random rnd = new Random();
            string alpha = "abcdefghijklmnopqrstuvwxyz";
            int letterInt;
            StringBuilder sb = new StringBuilder();

            for (int i = 0; i  best)
                {
                    Console.Write("The best guess so far is: ");
                    Console.WriteLine(guess);
                    best = correct;
                }
            }

            watch.Stop();
            TimeSpan ts = watch.Elapsed;
            Console.WriteLine("Success!");
            Console.Write("It took " + ts.TotalSeconds + " seconds for the sharp C to type ");
            Console.WriteLine("\"" + guess + "\"");

            Console.ReadLine();
        }
    }
}

Python:

```
import random
import time
#This

If you want performance, don't create objects in the inner loop:

static string generate(int len)
    {
        Random rnd = new Random();                    // creating a new object
        string alpha = "abcdefghijklmnopqrstuvwxyz";
        int letterInt;
        StringBuilder sb = new StringBuilder();      // creating a new object
        ....

    }

create them once and reuse them

StackExchange Code Review Q#42792, answer score: 50