So, its definitely there is a bug.

I tried 2 different versions of httrack on ubuntu/debian/osx.

There are always just 4 connections.
What is limiting it? How can i increase it?
If crawled site response very fast ( e.g. 0.01s ) - perforance with 4
connections is ok.
If crawled site response is average ( e.g. wikipedia ) - performance with 4
connections is very low.

Options like %N0 does not help.
I really don't understand how people use that software :/