Parallel apply is an important technique for achieving high speed replication and curing Replica lag. It works by spreading updates to Replicas over multiple threads that split transactions on each schema into separate processing streams. This in turn spreads I/O activity across many threads, which results in faster overall updates on the Replica. In ideal cases throughput on Replicas may improve by up to 5 times over single-threaded MySQL native replication.
It is worth noting that the only thing Tungsten parallelizes is applying transactions to Replicas. All other operations in each replication service are single-threaded.