Hi Sergei,
I don't know what it's so slow, but one reason could be that the web server of
that website runs on low specs. 
Even though you have a bandwidth of 100MB/s by your ISP, this doesn't mean you
can access a single website with that full bandwidth. 
I've seen in the forum, the last post is about informing the members about the
closing of that website, but I wasn't able to even open that page in the
web-browser. Some sort of overload information was displayed. I did not spent
much time on the website,  tried to reload that webpage, and finally the
website crashed with the following error response: 

"Warning: mysql_connect(): Too many connections in
/var/www/html/cms_admin/db_mysql.php on line 30"

So, the website crashed. The database server itself seems to be overwhelmed.
For me it looks really the bottle neck is on that website server respectively
even further, the DB connection, and not so much on your end respectively any
downloader tool.
I suggest, just let the job run. Not sure if there are other options right
now.
Check the results once in a while, if possible, to be sure that generally it's
ok, and then "only" a matter of time hopefully before it closes down.

Good luck!