What is the purpose of logical replication?
If you are using binary replication, the master and slave have to use the same major release of PostgreSQL. In other words, you cannot use streaming to upgrade from, say, PostgreSQL 10 to PostgreSQL 11 using a transaction log stream. Logical replication can help bridge this gap. In addition to that, logical replication can help selectively replicate data to various systems.
What is the performance impact of synchronous replication?
Synchronous replication will be slower than binary replication. In general, the performance impact of short transactions will be a lot larger than for long transactions. It is hard to state a precise number as the performance decrease depends significantly on network latency. The slower your network, the slower your synchronous replication.
Why not always use synchronous replication?
Try to use synchronous...
