# MacOS is able to limit bandwith form specific protocol or source-ip on the fly. Creating an bandwidth limtation rule won't break your download 

#Build a pipe with a limited bandwidth (example 1Mbits/s)
sudo ipfw pipe 1 config bw 1Mbit/s

#Assign a rule to redirect the protocol on that pipe (for example http from www.example.com)
sudo ipfw add 1 pipe 1 proto tcp src-port 80 src-ip www.example.com

#You can show your pipe like that
sudo ipfw pipe show

#Delete the created pipe with that both comands
# delete the redirection rule :
sudo ipfw del 1 
# delete the pipe :
sudo ipfw pipe 1 delete