I had to deal with an Excel xlsx file generated by a Payroll system. Not sure what is causing the slowness on column insert. Inserting a column into a 88,000 row file takes about 25 seconds.
I discovered that if I copy the entire worksheet to a new sheet as values and number formats, the insert column step will run almost instantly. The copy entire worksheet portion takes just 3 seconds! There are no formulas in the file nor conditional formatting.
This is the logic I used:
' Copy source worksheet
Dim rng As Range
Set rng = Worksheets("Sheet1").Cells
rng.Copy ' note that wks.Cells.Copy is very slow
' add new worksheet
Dim newWks As Worksheet
Set newWks = Sheets.Add(After:=ActiveSheet)
newWks.Name = "Values Only"
' paste values and number format into new worksheet
newWks.Range("A1").Select
' xlPasteValues is fast too
Selection.PasteSpecial Paste:=xlPasteValuesAndNumberFormats, Operation:=xlNone, SkipBlanks _
:=False, Transpose:=False
' if I also include a xlPasteFormats then the insert column will become very slow once again
' Selection.PasteSpecial Paste:=xlPasteFormats, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
' now the column insert is blazingly fast!
newWks.Columns(14).Insert
newWks.Cells(1, 14).Interior.ColorIndex = 35
' etc.