Very fast replication I have used the Streaming Replication guide on the PostgreSQL Wiki to configure replication between my two db servers.

But, when I updated a row on master server it is available on the slave instantly. It should be asynchronous, isn't it? What I am missing?

Servers are in separate places. The master's upload speed is ~75kb/sec. As far as I know, first of all, 16MB xlog file should be upload to slave, after that transactions available on slave. But it is happening in 1 sec. Why is it so fast?
Async and Slow are not the same thing. If you start putting real load on the replication stream, you will start to see delays, but not with a single test command.

From the PostgreSQL Wiki entry you linked to:


  In SR, XLOG records in partially-filled XLOG file are shipped too, implementing record-based log-shipping.'


So streaming replication is exactly that: The stream of updates is transported from master to slave.