Big excel file with vba, slow processing

Hi guys,

I've got a problem. I have an excel file with around 200,000+ rows with around 30 columns. This file contains functions such as IF, AND, OR as well as certain functions that I have written as there are certain cells that required my own formulas. Without any automation, the file size is around 12 mb.

I am trying to automate the entire process and now I have gotten up to around the 25th column and the process is starting to look really slow and bleak as the file size has increased to around 40mb.

Is there anyway by which I can make make the code more efficient? The current code I am using is:

   dim lastrow as long

    lastrow = Cells(Rows.Count, "B").End(xlUp).Row

    '~~> Change this to the relevant sheet name
    '~~> This is the sheet where the formula will go
    Set wsRawData = ThisWorkbook.Sheets("Raw Data")

    Set wsALD = ThisWorkbook.Sheets("AData")
    
    
    '---------This section begins Great Circle Distance in nm---------'
    
    With wsRawData

         Range("J6:J" & lastrow).Formula = "=GCDnm(D6,E6)"

----

I am pretty much a noob in VBA, but as you can see everytime I run this code, the GCDnm formula will be pasted all through the J column. This process is repeated for all the rows and different columns have different code. Is there a way by which I can do it in a more efficient way? i.e. everytime a new line is added onto the 200,000++ lines of data the code doesn't have to paste the formula for the entire column??

Sorry for the long post! Many thanks!