AutoRefresh takes 200ms (ish)

    var initialItems = Enumerable.Range(1, 5_000)
        .Select(i => new SelectableItem(i){IsSelected = true})
        .ToArray();

    var source = new SourceList<SelectableItem>();

    var filtered = source.Connect()
        .AutoRefresh(si=>si.IsSelected)
        .Filter(si => si.IsSelected)
        .Subscribe(_=> Console.WriteLine("got initial data"));

    var sw = Stopwatch.StartNew();
    source.AddRange(initialItems);
    sw.Stop();
    Console.WriteLine(sw.ElapsedMilliseconds);
    
--------------------------------------------------

  public class SelectableItem : AbstractNotifyPropertyChanged
    {
        public int Id { get; }

        public SelectableItem(int id)
        {
            Id = id;
        }

        private bool _isSelected;

        public bool IsSelected
        {
            get => _isSelected;
            set => SetAndRaise(ref _isSelected, value);
        }

    }