When I'm transferring large quantities of data using rsync, it would be helpful if I could have the average speed up until now at a glance, rather than a bunch of different speeds for each file.
Yes. Starting with rsync version 3.1.0 the
--info=progress2 argument will give you progress on the entire transfer, including speed of the entire transfer. You can see a little bit of detail on the rsync man page.
In case you're not running v3.1.0, rsync-3.0.9 takes the
--progress flag. The man page on my CentOS7 box reports
This option tells rsync to print information showing the progress of the transfer. This gives a bored user something to watch.