Trying to find a formula that QUICKLY counts errors, formula or results, on a SS

I am trying to just count errors in a defind range on my spreadsheet. I tried using an array formula but it killed the processing time. I then tried a UDF, shown below, but it also kills the processing time because it runs more than once when doing a standard calculation. Is there a way to set the formula to only run at the end of a calc cycle?

Function CountErrors()
Dim rng, c As Range, e As Integer

Set rng = ActiveSheet.UsedRange
    For Each c In rng
        If InStr(1, c.Formula, "#REF!") Or IsError(c) Then
            CountErrors = CountErrors + 1
        End If
    Next c
End Function

Thanks!