Where one side of the transfer is low powered (e.g. a home NAS) or where files are small, the time taken to compress, transfer and decompress is often greater than simply transferring uncompressed. One way round this is to have a threshold (by default say 50Kb) below which compression is not attempted, a command line switch taking a numerical arg like 100, 100K, 100M, 100G could permit the user to tune this.
The newest compression algorithms give you more options for a low-powered host. e.g., --cc=lz4. Hopefully that will be enough.