Macro slows down with no visible reason

Hi all,
I suffer strange problem with my macro, that I cannot explain by any google search.

My macro runs really slow and what is more interesting it's even slowing down during performance (as I turn on Application.ScreenUpdating, it is visible). I tried all possible "basic" solutions as ScreenUpdating, DisplayAlerts etc.

But, what is the most strange, it does work properly with another files.

I have five files with data, each refers to the folder, with "subfiles" used by end users. My macro just collects data from subfiles to main file. In four cases it runs in 15-20 seconds, in the fifth, more than 10 minutes.
Files are exactly the same: the only difference are the columns that are copied.
The size of the file (or subfiles) is also not the case - one of this 4 working files has more than 15000 rows, while this problematic less than 5000.

Have you any idea, what is wrong?

Thanks in advance,
Andrzej

Post the code, might just be a dodgy line doing too many calculations in certain cases.