Retrieve data from multiple workbooks without opening them - excel

Question

I've been struggling with the following code for the past weeks.
What it does is pretty simple, it allows me to retrieve data from multiple worksheets without opening them. The issue is that I have around 150 rows & 1700 columns of data to be filled thus around 255k cells of data... So it takes way too much time
The workbook looks like this:
Sub Worksheet_Change()
Dim Rng As Range
Dim r As Long
Dim s As String
Dim f As String
Dim i As Long
On Error GoTo ErrHandler
Dim m As Long
Application.ScreenUpdating = False
Application.EnableEvents = False
Application.AskToUpdateLinks = False
Application.DisplayAlerts = False
Application.DisplayStatusBar = False
m = Sheets("ECHEANCIER").Range("A" & Rows.Count).End(xlUp).Row
Range("I5:BMQ" & m).ClearContents
For r = 5 To m
For i = 9 To 1000
s = "'" & Range("B" & r).Value & "[" & Range("C" & r).Value & "]" & Range("D" & r).Value & "'!"
Sheets("ECHEANCIER").Cells(r, i).FormulaR1C1 = "=RC[-1]*(IF(ISNA(INDEX(" & s & " R1C1:R1000C1000,MATCH(R[" & 4 - r & "]C," & s & "C1,0),MATCH(RC7," & s & " R1,0)+1)),1,INDEX(" & s & " R1C1:R1000C1000,MATCH(R[" & 4 - r & "]C," & s & "C1,0),MATCH(RC7," & s & " R1,0)+1)))"
If Sheets("ECHEANCIER").Cells(r, i).Value = 0 Then
Sheets("ECHEANCIER").Cells(r, i).ClearContents
Exit For
End If
Range("I" & r & ":BMQ" & r).Copy
Range("I" & r & ":BMQ" & r).PasteSpecial Paste:=xlPasteValues
Next
Next
ExitHandler:
Application.EnableEvents = True
Application.ScreenUpdating = True
Application.AskToUpdateLinks = True
Application.DisplayAlerts = True
Application.DisplayStatusBar = True
Exit Sub
ErrHandler:
MsgBox Err.Description, vbExclamation
Resume ExitHandler
End Sub
I am also trying to having the formula already written instead of writting it VBA, but I understood that it is quite complicated and that you have to copy/paste as values in order to get the result but I don't get it.
Thanks a lot for your help!!!