I'm also with the same issue.

I'm on mac. installed through brew.

i'm using the following command:

httrack --continue --verbose --disable-security-limits -c8
--connection-per-second=8 --sockets=8 --max-rate=10000000  
<https://www.website.com/>  -O "/my/output/dir/"  -%v

and it only uses 1 active connection at 30kb/s