Compression is off by default in OpenSSH, at least `man 5 ssh_config` says:

> Specifies whether to use compression. The argument must be yes or no (the default).

So I'm surprised you see speedups with your invocation.

Good point. Seems like I enabled it in ~/.ssh/config ages ago and did forget about it. Nonetheless, it's good to check whether it's enabled when using rsync to transfer large, already well compressed files.