MySQL 5.7 introduced the LOGICAL_CLOCK type of multi-threaded slave (MTS). When using this type of parallel replication (and when slave_parallel_workers is greater than zero), slaves use information from the binary logs (written by the master) to run transactions in parallel. However, enabling parallel replication on slaves might not be enough to get a higher replication throughput (VividCortex blogged about such a situation recently in Solving MySQL Replication Lag with LOGICAL_CLOCK and Calibrated Delay). To get a faster slave with parallel replication, some tuning is needed on the master.
