Remove long running loop with someting more efficient

Hi... I'm trying to make my spreadsheets run more efficiently, and the first goal I have is replacing as many loops as I can with smarter code. Since I'm neither a programmer, nor all that bright, I'm anticipating some challenges doing so. But since I have more than 20 spreadsheets, each of which can be up to 1 million lines, the loops just aren't cutting it and I'd like to do it better.

So, the question is, how can I replace something like the code below, which loops through column H and if the active cell contains the value "XXX", the value from 2 columns to the left (F) are copied over?

Application.ScreenUpdating = False
Range("H2").Select

Do Until ActiveCell.Offset(0, -7).Value = ""
    If ActiveCell.Value = "" Then
       ActiveCell.Value = ActiveCell.Offset(0, -2).Value
    End If
    ActiveCell.Offset(1, 0).Select
Loop

I've tried using filters, but I can't seem to get to ONLY the visible cells - so I kind of gave up on that approach.

Thanks for any guidance!