C# Optimization - Performance degradation when using LINQ with a foreach

Employee.cs

namespace Engine
{
    public class Employee
    {
        public string Name { get; set; }
        public int Salary { get; set; }

        public Employee(string name, int salary)
        {
            Name = name;
            Salary = salary;
        }
    }
}

Test.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

using Engine;

namespace TestBench
{
    public partial class Test : Form
    {
        public Test()
        {
            InitializeComponent();
        }

        private void btnStart_Click(object sender, EventArgs e)
        {
            const int testIterations = 10;
            const int employees = 1000000;

            List<TestResult> testResults = new List<TestResult>();

            for(int i = 0; i < testIterations; i++)
            {
                testResults.Add(new TestResult(DurationForTest1(employees), DurationForTest2(employees)));
            }

            double averageDuration1 = testResults.Average(x => x.Test1.TotalMilliseconds);
            double averageDuration2 = testResults.Average(x => x.Test2.TotalMilliseconds);

            lblTimeSpan1.Text = averageDuration1.ToString();
            lblTimeSpan2.Text = averageDuration2.ToString();
            lblDifferencePercentage.Text = (((averageDuration2 - averageDuration1) / averageDuration1) * 100).ToString("N2") + " %";
        }

        private static List<Employee> BuildEmployeeList(int listSize)
        {
            List<Employee> employees = new List<Employee>();

            for(int i = 1; i <= listSize; i++)
            {
                employees.Add(new Employee(i.ToString(), i));
            }

            return employees;
        }

        private TimeSpan DurationForTest1(int listSize)
        {
            List<Employee> employees = BuildEmployeeList(listSize);

            int listMidPoint = listSize / 2;

            DateTime start = DateTime.Now;

            foreach(Employee employee in employees)
            {
                if(employee.Salary <= listMidPoint)
                {
                    employee.Salary = employee.Salary * 2;
                }
            }

            return (DateTime.Now - start);
        }

        private TimeSpan DurationForTest2(int listSize)
        {
            List<Employee> employees = BuildEmployeeList(listSize);

            int listMidPoint = listSize / 2;

            DateTime start = DateTime.Now;

            foreach(Employee employee in employees.Where(employee => employee.Salary <= listMidPoint))
            {
                employee.Salary = employee.Salary * 2;
            }

            return (DateTime.Now - start);
        }
    }
}

TestResult.cs

using System;

namespace Engine
{
    public class TestResult
    {
        public TimeSpan Test1 { get; set; }
        public TimeSpan Test2 { get; set; }

        public TestResult(TimeSpan test1, TimeSpan test2)
        {
            Test1 = test1;
            Test2 = test2;
        }
    }
}