Lazy vs no Lazy for IObservable pipeline construction

LazyVsNoLazy.cs

// by Theodor Zoulias
// https://stackoverflow.com/questions/72558093/in-rx-net-how-do-i-make-a-subject-to-resemble-taskcompletionsource-behavior/72561100?noredirect=1#comment128188846_72561100
//
// comment out "#define USING_LAZY"

#define USING_LAZY
using System;
using System.Reactive.Linq;
using System.Reactive.Subjects;
using System.Reactive.Threading.Tasks;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        const int count = 10_000;
        var mem0 = GC.GetTotalAllocatedBytes(true);
        for (int i = 0; i < count; i++)
        {
            var subject = new PromiseSubject<int>();
            var subscription = subject.Subscribe();
            subscription.Dispose();
        }
        var mem1 = GC.GetTotalAllocatedBytes(true);
        Console.WriteLine($"Allocated {(mem1 - mem0) / count:#,0} bytes per instance");
    }

    public class PromiseSubject<T> : ISubject<T>
    {
        private readonly TaskCompletionSource<(bool HasValue, T Value)> _tcs = new();

#if USING_LAZY
        private readonly Lazy<IObservable<T>> _lazyObservable;
#else
        private readonly IObservable<T> _observable;
#endif

        public PromiseSubject()
        {
#if USING_LAZY
            _lazyObservable = new(() =>
                _tcs.Task.ToObservable()
                .Where(r => r.HasValue)
                .Select(r => r.Value!));
#else
            _observable = _tcs.Task
                .ToObservable()
                .Where(r => r.HasValue)
                .Select(r => r.Value!);
#endif
        }

        public void OnCompleted() =>
            _tcs.TrySetResult((false, default!));

        public void OnError(Exception error) =>
            _tcs.TrySetException(error);

        public void OnNext(T value) =>
            _tcs.TrySetResult((true, value));

        public IDisposable Subscribe(IObserver<T> observer) =>
#if USING_LAZY
            _lazyObservable.Value.Subscribe(observer);
#else
            _observable.Subscribe(observer);
#endif
    }
}