VBA to write data in cells very slow - excel

Question

I have written a very straightforward script which writes data into excel cells.
Basically, this is a loop over an array and it writes data into specific cells or formulas.
The problem is that this part of the script is extremly slow.
Any ideas on how to improve thath?
Thanks.
For j = 0 To i - 1
'Insère nouvelle ligne
Rows(startRow & ":" & startRow).Select
Selection.Copy
Rows(startRow + 1 & ":" & startRow + 1).Select
Selection.Insert Shift:=xlDown
'Insère données
If roomType(j) <> "" Then
Feuil3.Cells(startRow, 1).Value = roomName(j)
Feuil3.Cells(startRow, 2).Value = roomSurface(j)
Feuil3.Cells(startRow, 7).Value = roomPeople(j)
Feuil3.Cells(startRow, 12).Value = roomPeople(j)
Feuil3.Cells(startRow, 5).Value = dict.Item(roomType(j))
Feuil3.Cells(startRow, 3).Value = roomHeight(j)
Feuil3.Range("F" & startRow).Formula = "=IFERROR(IF($E" & startRow & "=Data!$A$55,,ROUNDUP($B" & startRow & "/VLOOKUP($E" & startRow & ",Data!$A$3:$E$55,4,FALSE),0)),)"
Feuil3.Range("H" & startRow).Formula = "=$C$25"
Feuil3.Range("I" & startRow).Formula = "=IF($E" & startRow & "=Data!$A$55,$B" & startRow & "*$E$55,(MAX(F" & startRow & ",G" & startRow & ")*H" & startRow & "))"
Feuil3.Range("N" & startRow).Formula = "=IFERROR(VLOOKUP($K" & startRow & ",$M$22:$O$26,3,FALSE),)"
Feuil3.Range("O" & startRow).Formula = "=IFERROR(IF(ISBLANK(M" & startRow & ")=TRUE,L" & startRow & "*N" & startRow & ",L" & startRow & "*M" & startRow & "*N" & startRow & "),)"
Feuil3.Range("Q" & startRow).Formula = "=MAX(I" & startRow & ",O" & startRow & ")"
Feuil3.Range("T" & startRow).Formula = "=IFERROR(MAX(R" & startRow & ",S" & startRow & ")/(B" & startRow & "*C" & startRow & "),)"
End If
startRow = startRow + 1
Next j
Application.Calculation = xlCalculationAutomatic
Application.ScreenUpdating = True

Answer 1

It is quicker to assign an array to a range of cells than to assign to individual cells in a loop. You could try sometihing like: Worksheets("MySheet").Range("A1:D100").Value = myArray. It is because communication between VBA and Excel takes some time. A little bit more on that here: VBA Excel large data manipulation taking forever
Also using Cells instead of Range is aboout 2.6 times faster. Here is a question on that: Range() VS Cells() - run times

Answer 2

I cut the script in pieces.
This part takes most of the time
For j = 0 To i - 2
Feuil3.Cells(startRow, 5).Value = dict.Item(roomType(j))
Feuil3.Cells(startRow, 6).Formula = "=IFERROR(IF($E" & startRow & "=Data!$A$55,,ROUNDUP($B" & startRow & "/VLOOKUP($E" & startRow & ",Data!$A$3:$E$55,4,FALSE),0)),)"
Feuil3.Cells(startRow, 8).Formula = "=$C$25"
Feuil3.Cells(startRow, 9).Formula = "=IF($E" & startRow & "=Data!$A$55,$B" & startRow & "*$E$55,(MAX(F" & startRow & ",G" & startRow & ")*H" & startRow & "))"
Feuil3.Cells(startRow, 14).Formula = "=IFERROR(VLOOKUP($K" & startRow & ",$M$22:$O$26,3,FALSE),)"
Feuil3.Cells(startRow, 15).Formula = "=IFERROR(IF(ISBLANK(M" & startRow & ")=TRUE,L" & startRow & "*N" & startRow & ",L" & startRow & "*M" & startRow & "*N" & startRow & "),)"
Feuil3.Cells(startRow, 17).Formula = "=MAX(I" & startRow & ",O" & startRow & ")"
Feuil3.Cells(startRow, 20).Formula = "=IFERROR(MAX(R" & startRow & ",S" & startRow & ")/(B" & startRow & "*C" & startRow & "),)"
startRow = startRow + 1
Next j