Resilio Connect Agents do not compress data during transfer by default. Data compression can be enabled with tunnel_compress_stream custom profile parameter. The parameter must be set in Job Profile as custom parameter so that all the Agents involved implement compression for sure. If at least one Agent in the job has a different value for this parameter, the job will fail for it.
Resilio Connect Agents use gzip 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.