I find that for me at least apt-fast doesn't offer any speed improvements, I'm also not sure it is very fair.
Apt-fast is only really going to be helpful to you when the repo is bandwidth limited. Hitting an already hard worked server with multiple concurrent download requests may make your slice of the download pie a little faster but it isn't helpful for other downloaders or the host.
I have recently found another script that I find does offer some useful increase while spreading the load over multiple mirrors. apt-metalink
uses (and depends on) aria2 instead of wget/axel and will download a file from all the mirrors in sources.list or in sources.list.d/*.list It also depends on python-apt.
It's a bit rough at the moment and I'm not sure I'd trust it to handoff an actual dist-upgrade but I've been using it for a few weeks to handle the downloads.
apt-metalink dist-upgrade -d