using System;

using System.Collections.Generic;

using System.Diagnostics;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var list1 = new List<string>();

        var list2 = new List<string>();

        var list3 = new List<string>();

        for (int i = 0; i < 50000; i++)

        {

            list1.Add($"hello {i}");

            list2.Add($"hello {i}");

            list3.Add($"hello {i}");

        }

​

        var stopwatch = Stopwatch.StartNew();

        var results = list1.Concat(list2).Concat(list3);

        stopwatch.Stop();

        Console.WriteLine(stopwatch.Elapsed);

        stopwatch = Stopwatch.StartNew();

        list1.AddRange(list2);

        list1.AddRange(list3);

        stopwatch.Stop();

        Console.WriteLine(stopwatch.Elapsed);

    }

}