C# function call performance cost

FnPerfTest.cs

using System;
using System.Diagnostics;

using UnityEngine;

public interface InterfaceFunc {
    void InterfaceFn();
}

static public class MonoExtension {
    static public void ExtFunc(this MonoBehaviour mb) {}
}

public class FnPerfTest : MonoBehaviour, InterfaceFunc {
    private const int reps = 10000000;
    private string report;

    void Start() {
        Stopwatch stopwatch = new Stopwatch();

        StartTest(stopwatch);
        for (var i = 0; i < reps; ++i) {
            StaticFunction();
        }
        long staticFunctionTime = StopTest(stopwatch);

        StartTest(stopwatch);
        for (var i = 0; i < reps; ++i) {
            this.ExtFunc();
        }
        long extensionMethodTime = StopTest(stopwatch);

        StartTest(stopwatch);
        for (var i = 0; i < reps; ++i) {
            InstanceFunction();
        }
        long instanceFunctionTime = StopTest(stopwatch);

        StartTest(stopwatch);
        for (var i = 0; i < reps; ++i) {
            InterfaceFn();
        }
        long interfaceFunctionTime = StopTest(stopwatch);

        StartTest(stopwatch);
        for (var i = 0; i < reps; ++i) {
            VirtualFunction();
        }
        long virtualFunctionTime = StopTest(stopwatch);

        StartTest(stopwatch);
        Action lambda = () => {};
        for (var i = 0; i < reps; ++i) {
            lambda();
        }
        long lambdaTime = StopTest(stopwatch);

        StartTest(stopwatch);
        Action del = delegate{};
        for (var i = 0; i < reps; ++i) {
            del();
        }
        long delegateTime = StopTest(stopwatch);

        StartTest(stopwatch);
        // for (var i = 0; i < reps; ++i) {
        //     SendMessage("InstanceFunction");
        // }
        long sendMessageTime = StopTest(stopwatch);

        report = "Test,Time\n" +
            "Static Function," + staticFunctionTime + "\n" +
            "Extension Method," + extensionMethodTime + "\n" +
            "Instance Function," + instanceFunctionTime + "\n" +
            "Interface Function," + interfaceFunctionTime + "\n" +
            "Virtual Function," + virtualFunctionTime + "\n" +
            "Lambda," + lambdaTime + "\n" +
            "Delegate," + delegateTime + "\n" +
            "SendMessage," + sendMessageTime;
    }

    private void StartTest(Stopwatch sw) {
        sw.Reset();
        sw.Start();
    }

    private long StopTest(Stopwatch sw) {
        return sw.ElapsedMilliseconds;
    }

    void OnGUI() {
        GUI.TextArea(new Rect(0, 0, Screen.width, Screen.height), report);
    }

    public static void StaticFunction() {}
    public void InstanceFunction() {}
    public virtual void VirtualFunction() {}

    public void InterfaceFn() {}
}