It's hard to do much more without seeing an example of your data.
Can you post a sample file and try to be as specific as you can as to what you want done with it?

That last code I posted. It starts from the active cell as you requested, so you need to have a cell selected else it won't do anything.

I tested that code by filling a 4400x4400 range on a worksheet, selecting cell A1, and running the code.
All cells below the principal diagonal were deleted.

You can try for yourself. Like run the below testdata code on a blank sheet, making sure cell A1 is the active or selected cell. Then run my Post#11 code.

Doesn't it do just what you asked for? and fast enough? 
Sub testdata()
With Range("A1")
    .Resize(4400, 4400) = "axa"
    .Select
End With
End Sub