Slow Macro - Multiple Vlookups To External Sheets

I'm working on a macro which compares values in particular cells across different versions of a sheet. Unfortunately there are quite a lot of values, and my code is running very slowly.

I'm trying to get the data in a grid, with the file version across the top, and down the left the project name. For each file version I need to look up a specific value for every project listed.

At the moment, my macro is creating a seperate formula for each cell which does a vlookup on the project name to the specific external file.

The relevant bit of code is below

At the moment its runnig to slow by several orders of magnitude, Working on a 4 by 10 grid it takes about a minute, and I'm going to need it to be able to handle a 50 by 800 grid.

While the code is running, the CPU is not maxed out, so I'm assuming that its the calls to the file system which are taking all the time.

Application.ScreenUpdating = False

Dim ccount As Integer
Dim rcount As Integer
Dim sFilename As String
Dim sPath As String

With ActiveSheet
For ccount = 1 To Range("c1").End(xlToRight).Column - 2

sPath = Left(FileNamesList(ccount), InStr(FileNamesList(ccount), "Pipeline ~") - 1)

sFilename = Right(FileNamesList(ccount), Len(FileNamesList(ccount)) - InStr(FileNamesList(ccount), "Pipeline ~") + 1)

For rcount = 1 To Range("A65536").End(xlUp).Row - 2

by SDB.xls]Pipeline'!$A$1:$AO$300,3,0)

Cells(rcount + 2, ccount + 2) = "=VLOOKUP(""" & Cells(rcount + 2, 1) & """,'" & sPath & "[" & sFilename & "]Pipeline'!$A$1:$AO$300,3,0)"

Next rcount
Next ccount

End With