output.txt

Hello World!
A
Elapsed: 00:00:05.0107387
B
Elapsed: 00:00:02.5023560

Program.cs

using System;
using System.Diagnostics;
using System.Threading.Tasks;

namespace AsyncTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            MainAsync().GetAwaiter().GetResult();

            Console.ReadLine();
        }

        static async Task MainAsync()
        {
            Console.WriteLine("A");
            await A();

            Console.WriteLine("B");
            await B();
        }

        static async Task A()
        {
            var s1 = Stopwatch.StartNew();

            s1.Start();

            var a = await Foo();
            var b = await Foo();

            s1.Stop();

            Console.WriteLine("Elapsed: {0}", s1.Elapsed);
        }

        static async Task B()
        {
            var s1 = Stopwatch.StartNew();

            s1.Start();

            var aT = Foo();
            var bT = Foo();

            await Task.WhenAll(aT, bT);

            s1.Stop();

            Console.WriteLine("Elapsed: {0}", s1.Elapsed);
        }

        static async Task<int> Foo()
        {
            await Task.Delay(2500);
            return 1;
        }
    }
}