To fine-tune network throughput there are also tools such as cstream or mbuffer.
In addition to
-t num to throttle data throughput to
num bytes per second,
cstream, for example, not only has a
-b num option to set the block size used for read/write, but also a
-B num option to buffer an input block up to
num bytes before writing.
As an alternative to
rsync you could try
nc (netcat) or
ncat, especially if the data stream is not going to go over the internet.
ncat is part of the Nmap Project and has SSL support though.
# netcat + tar + cstream example to throttle network throughput
# requires netcat on both servers
# cf. http://superuser.com/questions/291803/best-way-to-copy-millions-of-files-between-2-servers/
# cstream -T 10 -t 4m </dev/urandom >/dev/null # test
nc -l -p 2342 | tar -C /target/dir -xzf - # destination box
tar -cz /source/dir | cstream -T 30 -t 4m | nc Target_Box 2342 # source box