Why the chart becomes so slow?

I just modified the Sample SMA a little to change the PlotType to Histogram, then the chart became very slow. Could anyone please tell me what happened and how to deal with it? Thank you!

using System;
using cAlgo.API;
using cAlgo.API.Internals;
using cAlgo.API.Indicators;
using cAlgo.Indicators;

namespace cAlgo
{
    [Indicator(IsOverlay = true, TimeZone = TimeZones.UTC, AutoRescale = false, AccessRights = AccessRights.None)]
    public class SMAHistogram : Indicator
    {
        [Parameter("Source")]
        public DataSeries Source { get; set; }

        [Parameter(DefaultValue = 20)]
        public int Periods { get; set; }

        [Output("Main", PlotType = PlotType.Histogram, Color = Colors.White, Thickness = 1)]
        public IndicatorDataSeries Result { get; set; }

        public override void Calculate(int index)
        {
            double sum = 0.0;

            for (int i = index - Periods + 1; i <= index; i++)
            {
                sum += Source[i];
            }
            Result[index] = sum / Periods;
        }
    }
}