Testing the cost of boxing in System.Reactive Observables

Question

I was curious whether boxing impacts performance in a long chain of IObservable<T> operators where T is a value type, and whether performance can be improved by changing T to Box<T> from CommunityToolkit.HighPerformance. I tested this in the form of some unit tests. I also included a test without the long chain of operators to establish how much of the runtime is due to those operators.

The results I got were so surprising that I question whether I'm testing what I think I'm testing. The test where T is a bare value type runs fastest. This is the opposite of what I expected. Using Box<T> actually hurts performance, with runtime similar to an ordinary reference type. I get similar results when I run all the tests in a batch or run each one cold.

Do my tests actually demonstrate that T being a value type is faster, at least for these types?

    [TestClass]
    public class PerformanceTests
    {
        private const int RepeatCount = 10000000;
        private const int DoCount = 100;

        [TestMethod]
        public void TestReferenceType()
        {
            Observable.Repeat(new object(), RepeatCount).EmptyDo(DoCount).Subscribe(o => { });
        }

        [TestMethod]
        public void TestValueTypeBaseline()
        {
            Observable.Repeat(0, RepeatCount).Subscribe(o => { });
        }

        [TestMethod]
        public void TestValueType()
        {
            Observable.Repeat(0, RepeatCount).EmptyDo(DoCount).Subscribe(o => { });
        }

        [TestMethod]
        public void TestBoxT()
        {
            Observable.Repeat(Box<int>.GetFrom(0), RepeatCount).EmptyDo(DoCount).Subscribe(o => { });
        }
    }

    internal static class RxExtensions
    {
        public static IObservable<T> EmptyDo<T>(this IObservable<T> source, int count)
        {
            for(int i = 0; i < count; ++i)
            {
                source = source.Do(o => { });
            }
            return source;
        }
    }

Results:

Test	Time
TestValueTypeBaseline	1.7s
TestValueType	7.2s
TestReferenceType	8.8s
TestBoxT	8.8s