The synchronize_sql extension checks out a connection from the pool while generating an SQL string. In cases where a connection is necessary in order to properly escape input, and multiple inputs in the query need escaping, this can result in fewer connection checkouts and better overall performance. In other cases this results in a performance decrease because a connection is checked out and either not used or kept checked out longer than necessary.
The adapters where this extension may improve performance include amalgalite, mysql2, postgres, jdbc/postgresql, and tinytds. In these adapters, escaping strings requires a connection object for as proper escaping requires calling an escaping method on the connection object.
This extension is most helpful when dealing with queries with lots of strings that need escaping (e.g. IN queries with long lists). By default, a connection will be checked out and back in for each string to be escaped, which under high contention can cause the query to spend longer generating the SQL string than the actual pool timeout (since every individual checkout will take less than the timeout, but the sum of all of them can be greater).
This extension is unnecessary and will decrease performance if the single threaded connection pool is used.