VBA code bogging down computer during execution - vba

Question

The code below was designed to run through sheets in a workbook and carry out calculations based on headers found. The issue that I am having is that the workbook contains sometimes up to 50 sheets of data with (at times) 2000 lines. This bogs the computer down during execution of the code.
Is there a way to optimize this code so that the computer would not be bogged down or am I limited by using VBA for this process? Thanks!
Here is the code:
Option Explicit
Sub ReturnMarginal()
Dim ws As Worksheet
Dim lngLowLimCol As Long, strLowLimCol As String
Dim lngHiLimCol As Long, strHiLimCol As String
Dim lngMeasCol As Long, strMeasCol As String
Dim lngLastRow As Long
Dim wsf As WorksheetFunction
' get worksheetfunction references
Set wsf = Application.WorksheetFunction
' iterate worksheets
For Each ws In ThisWorkbook.Worksheets
' validate LowLimit label is on sheet
If Not (ws.Rows(1).Find("LowLimit") Is Nothing) Then
' get location of input data columns and number of rows
lngLowLimCol = wsf.Match("LowLimit", ws.Rows(1), 0)
lngHiLimCol = wsf.Match("HighLimit", ws.Rows(1), 0)
lngMeasCol = wsf.Match("MeasValue", ws.Rows(1), 0)
lngLastRow = ws.Cells(1, lngLowLimCol).End(xlDown).Row
' get column letters for input data columns
strLowLimCol = Split(ws.Cells(1, lngLowLimCol).Address(True, False), "$")(0)
strHiLimCol = Split(ws.Cells(1, lngHiLimCol).Address(True, False), "$")(0)
strMeasCol = Split(ws.Cells(1, lngMeasCol).Address(True, False), "$")(0)
' output headers
ws.Range("P1") = "Meas-LO"
ws.Range("Q1") = "Meas-Hi"
ws.Range("R1") = "Min Value"
ws.Range("S1") = "Marginal"
' assign formulas to outputs
' Meas-LO
'Range("P2:P" & lngLastRow).Select
' With Selection
' Selection.NumberFormat = "General"
' .Value = .Value
' End With
With ws.Range("P2:P" & lngLastRow)
.Formula = "=IF(" & strLowLimCol & "2" = ""---"," &
strMeasCol & "2-" & strLowLimCol & "2," & _
9999)"
End With
' Meas-Hi
With ws.Range("Q2:Q" & lngLastRow)
.Formula = "=IF(ISNUMBER(" & strHiLimCol & "2)," & _
strMeasCol & "2-" & strHiLimCol & "2," & _
9999 & "2)"
'strMeasCol & "2)"
End With
' Min Value
With ws.Range("R2:R" & lngLastRow)
.Formula = "=MIN(P2,Q2)"
End With
' Marginal
With ws.Range("S2:S" & lngLastRow)
.Formula = "=IF(AND(R2>=-3,R2<=3),""Marginal"",R2)"
End With
End If
Next ws
End Sub