When set to not apply compression, writing compressed output should mostly just amount to memcpy'ing the input data to the output. However, actual performance seem to be on the order of 120 MB/s. Some quick profiling indicates that a substantial amount of time is spent updating hash chains and recording literals. Neither of these operations should be necessary to produce uncompressed output.

Benchmarking via flate2:

let mut encoder = flate2::write::ZlibEncoder::new(Vec::new(), flate2::Compression::new(0));
encoder.write_all(&bytes).unwrap();
encoder.flush_finish().unwrap();