The PostgreSQL driver buffer all rows in the result set before handing it over to DbVisualizer. To solve this 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.