Performance issue with displaying total amount at the footer

Hello Community,

I have customized the Relase AR Documents screen and showing the total (Amount column) at the Footer but screen is taking long time to load and gettting performance issue. Can you please review and help me with the code changes?

 public class ARDocumentReleaseNB_Extension : PXGraphExtension<PX.Objects.AR.ARDocumentRelease>
  {
    #region Event Handlers

    protected virtual void BalancedARDocument_Availability_FieldSelecting(PXCache cache, PXFieldSelectingEventArgs e)
  {
      BalancedARDocument row = e.Row as BalancedARDocument; 
      e.ReturnValue = string.Concat("Total Amount: ", +Math.Round(Convert.ToDecimal(Base.ARDocumentList.Select().FirstTableItems.ToList().RowCast<BalancedARDocument>().Select(x => x.CuryOrigDocAmt).Sum()), 2));

  }