Why is this an issue?

Using Thread.Sleep in a test might introduce unpredictable and inconsistent results depending on the environment. Furthermore, it will block the thread, which means the system resources are not being fully used.

[TestMethod]
public void SomeTest()
{
    Thread.Sleep(500); // Noncompliant
    // assertions...
}

An alternative is a task-based asynchronous approach, using async and await.

More specifically the Task.Delay method should be used, because of the following advantages:

It is asynchronous: The thread will not be blocked, but instead will be reused by other operations
It is more precise in timing the delay than Thread.Sleep
It can be canceled and continued, which gives more flexibility and control in the timing of your code

[TestMethod]
public async Task SomeTest()
{
    await Task.Delay(500);
    // assertions...
}

Another scenario is when some data might need to be mocked using Moq, and a delay needs to be introduced:

[TestMethod]
public void UserService_Test()
{
    var userService = new Mock<UserService>();
    var expected = new User();

    userService
        .Setup(m => m.GetUserById(42))
        .Returns(() =>
        {
            Thread.Sleep(500); // Noncompliant
            return Task.FromResult(expected);
        });

    // assertions...
}

An alternative to Thread.Sleep while mocking with Moq is to use ReturnsAsync and pass the amount of time to delay there:

[TestMethod]
public void UserService_Test()
{
    var userService = new Mock<UserService>();
    var expected = new User();

    userService
        .Setup(m => m.GetUserById(42))
        .ReturnsAsync(expected, TimeSpan.FromMilliseconds(500));

    // assertions...
}

Why is this an issue?

Resources

Documentation