PostgreSQL Replication delay debugging
Maybe you are experienceing what is described at the end of the following article:

http://www.niwi.be/2013/02/16/replication-status-in-postgresql/


  In a very busy database, with many writes per second, this number will remain fairly accurate. However, in a system where there are few writes, the "replication_delay" will continually grow because the last replayed transaction timestamp isn't increasing (this is generally the same limitation as MySQL's SHOW SLAVE STATUS output).