Toggle Button To Fill In Blank Cells With N/A

Hello,

I have the attached spreadsheet where I need to fill in any blank cells with "N/A".
The range is quite large, so with the current code, it takes a long time to run.

Is there any way to make this more efficient?:

Private Sub Toggle_NA_Click()

Dim TestResults As Range, c5 As Range
Set TestResults = Range("A3:X63")

Application.EnableEvents = False
Application.ScreenUpdating = False

If Toggle_NA.Value = True Then
    For Each c5 In TestResults
        If c5.Value = "" Then c5.Value = "N/A"
    Next c5
Else
    For Each c5 In TestResults
        If c5.Value = "N/A" Then c5.Value = ""
    Next c5
End If

Application.EnableEvents = True
Application.ScreenUpdating = True

End Sub

Many Thanks!
Storm08