Calendar taking a long time to switch days in Day view

Most likely this is caused by loading more events than necessary. You can try limiting the event set using StartDate and Days properties.

See an example from the ASP.NET MVC 4 event calendar tutorial (http://code.daypilot.org/65402/event-calendar-for-asp-net-mvc-4-razor):

protected override void OnFinish()
{
  // update was not requested in the event handler using Update() or UpdateWithMessage()
  // skip event reloading
  if (UpdateType == CallBackUpdateType.None)
  {
    return;
  }

  Events = new EventManager().FilteredData(StartDate, StartDate.AddDays(Days)).AsEnumerable();

  DataStartField = "eventstart";
  DataEndField = "eventend";
  DataTextField = "name";
  DataIdField = "id";
}

Data-loading method:

public class EventManager
{
  public DataTable FilteredData(DateTime start, DateTime end)
  {
    SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM [event] WHERE NOT (([eventend] <= @start) OR ([eventstart] >= @end))", ConfigurationManager.ConnectionStrings["daypilot"].ConnectionString);
    da.SelectCommand.Parameters.AddWithValue("start", start);
    da.SelectCommand.Parameters.AddWithValue("end", end);

    DataTable dt = new DataTable();
    da.Fill(dt);

    return dt;
  }
}