TestMagnitude

// Make sure to use this in your namespace declarations
using System.Diagnostics;
// This is needed because System.Diagnostics also contains a Debug class
using Debug = UnityEngine.Debug;

// We'll need a lot of iterations to get any result for this.
public int iterations = 1000000;

void TestMagnitude()
{
    // Fill up an array of vectors to test on.
    Vector3[] testVectors = new Vector3[iterations];
    for (int i = 0; i < iterations; i++)
    {
        testVectors[i] =
            new Vector3(
                Random.Range(-100, 100),
                Random.Range(-100, 100),
                Random.Range(-100, 100));
    }

    // Instantiate a new stopwatch
    Stopwatch stopwatch = new Stopwatch();

    // Set up our value outside of the for loop just in case there's any
    // performance hit from declaring it inside the loop.
    float value;

    // Test sqrMagnitude on our test vectors
    stopwatch.Start();
    for (int i = 0; i < iterations; i++)
    {
        value = testVectors[i].sqrMagnitude;
    }
    stopwatch.Stop();

    // "Save" the total test time in a variable to output later.
    double sqrMagnitudeTime = stopwatch.ElapsedMilliseconds;
    // Make sure to reset/clear the stopwatch befor using it again.
    stopwatch.Reset();

    // Test magnitude on our test vectors
    stopwatch.Start();
    for (int i = 0; i < iterations; i++)
    {
        value = testVectors[i].magnitude;
    }
    stopwatch.Stop();

    double magnitudeTime = stopwatch.ElapsedMilliseconds;
    stopwatch.Reset();

    // Output the test results to the console.
    Debug.LogFormat(
        "Magnitude test results:\n" +
        "total sqrMagnitude time: {0:G}ms\n" +
        "average sqrMagnitude time: {1:G}ms\n" +
        "total magnitude time: {2:G}ms\n" +
        "average magnitude time: {3:G}ms\n",
        sqrMagnitudeTime,
        (sqrMagnitudeTime / iterations),
        magnitudeTime,
        (magnitudeTime / iterations)
        );
}