There's an issue in the ArcGISRuntime for Xamarin.Forms. When a map is created with a GroupLayer and some child layers, using the back button to go to a different page on a poor connection can cause frequent freezes usually lasting for over a minute. I've found that removing the GroupLayer makes the issue go away, but I'd like to be able to use GroupLayers.
A demonstration of this issue can be found here in the iOS project under the title "Back & Freeze Bug".