> the program has free download slots.  How come it can not 
> use those free slots do download these files 
immediately?  

Because there are many files to be downloaded before. 
HTTrack alwas scans and download in sequential phases, 
using a heap of links.

You might want to reduce the mirror scope, to get faster 
the files you want, using scan rules (Set Options /Scan 
rules), using something like:
-* +www.example.com/*thefolderIwant*