On zooming in, there is no change in performance. The map loads equally as slow as if trying to always render all 200k map points.
I have checked that the shapefile does have a spatial index (.shx) file (just not sure if really loaded). I have set the min & max scale, simple symbology and intentionally set a definition expression to show only 1 point but no effect on performance.
FeatureLayer flayer = new FeatureLayer(shapefile)
{
ID = LayerID,
DisplayName = path,
Opacity = 0.6,
Renderer = new SimpleRenderer { Symbol = new SimpleMarkerSymbol { Color = Colors.CornflowerBlue, Size = 12, Outline = new SimpleLineSymbol { Color = Colors.Black, Width = 1 } } },
MaxScale = 400,
MinScale = 100000000,
DefinitionExpression = "ID = 17020001003930",
};