If you set SelectedDate when the Calendar control is created, does not take any effect in the UI on iOS.
As a workaround, set SelectedDate at a later stage.
The more appointments you add to the AppointmentsSource, the poorer the performance of the Calendar is. This can be observed when switching months in month view. In case you have a huge amount of appointments, switching the months takes time. The issue is specifically obvious when a single month contains lots of appointments(600 for example). However, it is present when you are switching months that do not have any appointments at all as well.
In a scenario where you are trying to set the AppointmentsSource of the RadCalendar in the OnAppearing event of the Page, a NullReferenceException is thrown in the Telerik.XamarinForms.InputRenderer.WinRT.CalendarAdapter UpdateAppointmentsSource method. A workaround is to make the OnAppearing method async and await the action that sets the AppointmentSource.