There is no Speed up function in HTTrack,  ther is alot of SLOW DOWN, so that
you DON'T ABUSE the poor web server your ripping everything off of.

You could limit your self to only a subpart of the site and not grab the
entire thing.  But other than that just let it run over night.  If it takes
more that 11 hours your doing something wrong, or grabbing way to much, or
HTTrack has crash due to low memory