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".
Thanks for sharing the repro project - I see it references Esri.ArcGISRuntime.Xamarin.Forms Version 100.11.2 - are you able to repro with the current release? (100.15)
e.g. we fixed an issue with nested group layers for 100.15.
I tried with version 100.15 and I could still repro the issue.