If you are having to use rsync, and transferring a lot of data. I find -z (--compress) is a great bandwith and time saver.
But the caveat is compressing already compressed data. It does not make sense to (re)compress. Therefore '--skip-compress=' is to the rescue.
The rsync man pages say, the list needs to be seperated by forward slashes (/). I dont know why forward slashes, when pretty much every other script / program makes use of commas.
Here is a list I found and all you need to do is:
rsync -vaz --skip-compress=$RSYNC_SKIP_COMPRESS /source /target