Performance collapse with many Refresh calls and zooming out

[pauldendulk]

Mapsui Version
Current master

Mapsui Platform
Tested on WPF where it was severe. Also on Avalonia where it was less severe but still clearly there.

Device
Windows Desktop

Describe the bug
The application becomes extremely sluggish when maximally zoomed out.

To Reproduce
When running the sample pasted below and zooming out so that only one tile is rendered performance significantly decreases. Even though less needs to be rendered (just one tile instead of thirty-two).

using Mapsui.Extensions;
using System;
using System.Threading.Tasks;
using System.Timers;
using Mapsui.Tiling;
using Mapsui.Widgets.InfoWidgets;

namespace Mapsui.Samples.Common.Maps.Geometries;

public sealed class ManyRefreshesSample : ISample, IDisposable
{
    private bool _disposed;

    public string Name => "Many Refreshes";
    public string Category => "Performance";

    private readonly Timer _timer = new(1);

    public Task<Map> CreateMapAsync()
    {
        var map = new Map();

        map.Layers.Add(OpenStreetMap.CreateTileLayer());
        map.Navigator.ZoomToBox(map.Layers[0].Extent);
        map.Widgets.Add(new MapInfoWidget(map));

        InitializeTimer(_timer, map);

        return Task.FromResult(map);
    }

    private static void InitializeTimer(Timer timer, Map map)
    {
        var random = new Random(38445);
        timer.Elapsed += (sender, args) => TimerElapsed(map);
        timer.Start();
    }

    private static void TimerElapsed(Map map)
    {
        for (var i = 0; i < 1000; i++)
            map.Refresh();
    }

    public void Dispose()
    {
        if (_disposed)
            return;

        _timer.Dispose();

        _disposed = true;
    }
}

[pauldendulk]

Analysis

It is counter intuitive that the application gets slower when only one tile needs to be rendered. It should be faster. In fact when zooming out maximally the application hardly responds at all.

I ran all kinds of analysis tools and custom configurations. What seems to be happening is that because everything is faster there are more calls to start and stop the FetchWorkers (which rapidly conclude that there is nothing to fetch). I think the rapid creation and deletion of threads within the worker put a high load on the garbage collector and this disturbed other processes which cascaded things further.

[pauldendulk]

I updated the sample to the most minimal things needed to reproduce the problem. The essential thing is the high number of refresh calls. They are now at 1 million per second. That is pretty extreme and you might argue that this would cause problems in any software, but the remarkable thing is that is does not cause problems at zoom level 5 to 20, but it does at level 1 when rendering takes less time.