Even though there is no implemention of Mapsui in Xamarin.Forms you can use it. This is also possible with little effort and good performance. But what is Mapsui? Mapsui is an open source project lead by Paul den Dulk. It is available via NuGet and Github. Up to now the project supports WPF as well as Xamarin Nativ Android, iOS and UWP. It allows you to use OpenStreetMap or your own map data both online and offline. The map data is usually in the *.mbtiles format. Furthermore, you can place markers, labels and other shapes on the map. The performance is pretty good. In my first project with this library I could place over 1500 markers on a 300 MB large map without any noticeable performance issues.
The usage of the library itself is pretty simple and you can get some samples for Mapsui on Github. The WPF application shows every aspect of the library at once. The native Xamarin apps load only one aspect, which can be set in the sample code. For example you can switch from your custom map data to OpenStreetMap and so on.
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