The PostgreSQL driver buffer all rows in the result set before handing it over to DbVisualizer.
To solve this in DbVisualizer 10.0.21 and later versions, open the connection Properties tab and then select the database type category (PostgreSQL, MySQL, Oracle, etc) category. In the right pane there is the JDBC Fetch Size setting, set it to 10, click Apply and re-connect.
To solve this in DbVisualizer 10.0.20 and older versions, add the following driver property in Connection Properties / Driver Properties by adding a new row (parameter = value):
java.sql.statement.setFetchSize = 10
Click Apply and re-connect.
In the SQL Commander, if you are running a @export script you must also make sure that Auto Commit is off, as the driver still buffers the result set in auto commit mode.