Hi Leo,
thanks. This thread (Copy files over network - Dopus slower than Windows Explorer) also led me to investigate deeper.
It looks like since I have two network cards in the server I had some TCP congestion issue. Only one card active now and it's much better. However, I performed some tests and captured everything via stopwatch and Process Monitor while copying with Dopus, Total Commander and Windows Explorer.
Windows Explorer performs best of all the three. I can send you logs of you want.
It's a 475 MB compressed ZIP file and I spent some hours to collect all the info. Would appreciate if you look into this further.