SelectMany performance

Output.txt

Compiling the source code....
$mcs main.cs -out:demo.exe 2>&1
main.cs(37,13): warning CS0219: The variable `ids1' is assigned but its value is never used
main.cs(40,13): warning CS0219: The variable `ids1' is assigned but its value is never used
Compilation succeeded - 2 warning(s)

Executing the program....
$mono demo.exe 
SelectMany method chain : 00:00:00.7027228
SelectMany linq         : 00:00:00.6438793
Nested foreach          : 00:00:00.3397560
AddRange                : 00:00:00.0498781

Program.cs

using System.IO;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics;

class Program {
    public class PaymentItemViewModel {
        public List<int> Placements { get; set; }

        public int StandardPayment { get; set; }
    }
    
    static void Main() {
        var numbers = Enumerable.Range(0, 4000).ToList();
        var models = numbers.Select(n => new PaymentItemViewModel() { 
            Placements = numbers,
            StandardPayment = n
        }).ToList();
        
        Test("SelectMany method chain", ()=>Test4(models));
        Test("SelectMany linq        ", ()=>Test1(models));
        Test("Nested foreach         ", ()=>Test2(models));
        Test("AddRange               ", ()=>Test3(models));
    }
    
    static void Test(string name, Action test)
    {
        var stopwatch = new Stopwatch();
        stopwatch.Start();
        test();
        stopwatch.Stop();
        Console.WriteLine(name + " : " + stopwatch.Elapsed);
    }
    
    static void Test1(List<PaymentItemViewModel> models) {
        var ids1 = models.SelectMany(m => m.Placements).ToList();
    }
    static void Test4(List<PaymentItemViewModel> models) {
        var ids1 = (from model in models
                    from placement in model.Placements
                    select placement).ToList();
    }
    static void Test2(List<PaymentItemViewModel> models) {
        var ids2 = new List<int>();
        foreach(var model in models) {
            foreach(var placement in model.Placements) {
                ids2.Add(placement);
            }
        }
    }
    static void Test3(List<PaymentItemViewModel> models) {
        var ids3 = new List<int>();
        foreach(var model in models) {
            ids3.AddRange(model.Placements);
        }
    }
}