VBA macro to convert cell Function into text expression is very slow

I am trying to set ' in front of every formula so that the function is not evaluated and is displayed as plain text.

Sub ConvertFormulasToTextForSheet()
  Dim ws As Worksheet
  Dim rng As Range
  Dim cell As Range
  Dim sheetName As String
 
  sheetName = "Sheet1" 

  Set ws = ThisWorkbook.Worksheets(sheetName)

  Set rng = ws.UsedRange

  For Each cell In rng
      cell.Value = "'" & cell.Formula

  Next cell

End Sub

executing that function is extremly slow, it basically convert 3-4 per seconds.
Is there any way how to speed that up?

Use an array:

Dim vals As Variant
vals = ws.UsedRange.Formula

If IsArray(vals) Then
    Dim i As Long, j As Long
    For i = LBound(vals, 1) to UBound(vals, 1)
        For j = LBound(vals, 2) to UBound(vals, 2)
            vals(i, j) = "'" & vals(i, j)
        Next
    Next
Else
    vals = "'" & vals
End If

ws.UsedRange.Value = vals

Arrays are much more efficient that iterating over each formula. Setting Application.Calculation = xlCalculationManual will also give the code a performance boost.
I added Suspend as a parameter. This will make it easy to enable and disable the formulas.

Usage:
> SuspendFormula ActiveSheet, True
> SuspendFormula ActiveSheet, False

Code:

Sub SuspendFormula(Worksheet As Worksheet, Suspend As Boolean)
    Dim InitialCalculationState As XlCalculation
    Application.ScreenUpdating = False
    InitialCalculationState = Application.Calculation
    Application.Calculation = xlCalculationManual
        
    Dim Target As Range
    Set Target = Worksheet.UsedRange
    Dim Data As Variant
        
    If Target.Count = 1 Then
        ReDim Data(1 To 1, 1 To 1)
        Data(1, 1) = Target.Formula
    Else
        Data = Target.Formula
    End If
        
    Dim r As Long, c As Long
        
    For r = 1 To UBound(Data)
        For c = 1 To UBound(Data, 2)
            If Suspend Then
                If Left(Data(r, c), 1) = "=" Then Data(r, c) = "'" & Data(r, c)
            Else
                If Left(Data(r, c), 2) = "'=" Then Data(r, c) = Mid(Data(r, c), 2)
            End If
        Next
    Next
        
    Target.Formula = Data
    
    Application.Calculation = InitialCalculationState
End Sub