Working with ranges

Hi to all.
I'm changing the formulas using a macro, but this process is very slow (around 4 mins). My ranges are not huge, lets say "B8:B91,D8:D91,F8:F91"
and I'm using the REPLACE command to change this formulas:

Sheets("sheet1").Range("B8:B91,D8:D91,F8:F91").Replace _
            What:="$A" , Replacement:="$D", LookAt:=xlPart, _
                SearchOrder:=xlByColumns, MatchCase:=False

But if I use:

Sheets("sheet1").Range("B8:B91").Replace _
            What:="$A" , Replacement:="$D", LookAt:=xlPart, _
                SearchOrder:=xlByColumns, MatchCase:=False
Sheets("sheet1").Range("D8:D91").Replace _
            What:="$A" , Replacement:="$D", LookAt:=xlPart, _
                SearchOrder:=xlByColumns, MatchCase:=False
Sheets("sheet1").Range("F8:F91").Replace _
            What:="$A" , Replacement:="$D", LookAt:=xlPart, _
                SearchOrder:=xlByColumns, MatchCase:=False

"seems" to be more fast.

I'm using too

With Application
  .DisplayAlerts = False
  .AskToUpdateLinks = False
  .ScreenUpdating = False
  .Calculation = xlCalculationManual
  .EnableEvents = False
End With

How can I improve the speed of this process?