Resilio Connect Agents don't compress data during transfer by default. To enable compression, set the tunnel_compress_stream parameter in the Job Profile. Ensure all Agents in the job have this parameter set; otherwise, the job will fail.
Resilio Connect Agents use zlib level 6 algorithm to compress data pieces.
It can be useful in case you have extremely limited (or expensive) connection and high-profile CPUs on all the peers participating the transfer.