Laggy slave OK to have only one system user in processlist?
Having replication with only one system user sounds a lot like MySQL 4.1 replication. It was single threaded. That does not sound anything like MySQL 5.x replication.

When you run SHOW SLAVE STATUS\G, you will see something like this:

mysql&gt; SHOW SLAVE STATUS\G
*************************** 1. row ***************************
             Slave_IO_State: Waiting for master to send event
                Master_Host: 10.48.20.253
                Master_User: replicant
                Master_Port: 3306
              Connect_Retry: 60
            Master_Log_File: mysql-bin.000262
        Read_Master_Log_Pos: 803779735
             Relay_Log_File: relay-bin.067799
              Relay_Log_Pos: 402744
      Relay_Master_Log_File: mysql-bin.000262
           Slave_IO_Running: Yes
          Slave_SQL_Running: Yes
            Replicate_Do_DB:
        Replicate_Ignore_DB:
         Replicate_Do_Table:
     Replicate_Ignore_Table:
    Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
                 Last_Errno: 0
                 Last_Error:
               Skip_Counter: 0
        Exec_Master_Log_Pos: 803779735
            Relay_Log_Space: 402598
            Until_Condition: None
             Until_Log_File:
              Until_Log_Pos: 0
         Master_SSL_Allowed: No
         Master_SSL_CA_File:
         Master_SSL_CA_Path:
            Master_SSL_Cert:
          Master_SSL_Cipher:
             Master_SSL_Key:
      Seconds_Behind_Master: 0
1 row in set (0.00 sec)


Please note the two lines

           Slave_IO_Running: Yes
          Slave_SQL_Running: Yes


When you see Yes to both of these lines, there must exist two process IDs with system user as the user. If you only see one, then either the IO thread (network connection back to the Master) is gone, or the SQL thread (load thread to read relay logs) encountered a MySQL error.

Thread States for Replication


IO is Yes and SQL is Yes : Replication should be running
IO is No and SQL is Yes : Network Connection to Master is gone
IO is Yes and SQL is No : SQL Error reading from Relay Logs
IO is No and SQL is No


Someone or Something ran STOP SLAVE;
MySQL started up with skip-slave-start configured



If both Slave_IO_Running and Slave_SQL_Running are Yes and Relay_Log_Space keeps on increasing, then my guess would be that LOAD DATA INFILE was executed on the Master and all the data is being shipped through the relay logs as a single unit (See my post MySQL 5.6 showing wrong &quot;second_behind_master&quot;).

The only other thing that can cause


Slave_IO_Running to be Yes
Slave_SQL_Running to be Yes
Relay_Log_Space to be be increasing


Would be a very heavy write transaction with row-based replication executed as a group commit.

UPDATE 2014-11-17 17:30 EST

Based on the chat room conversation, we have discussed


Over 2TB of InnoDB data, so setting up replication from scratch is out of the question
Discovered that binary logging is enabled on the Slave
I proposed that binary logging on the Slave maybe causing the lag


I recent encountered this with a client
They lag one extra second every two seconds
I disabled binary logging on the Slave and it worked

My suggestion for this post is  to disable binary logging on the Slave like this


STOP SLAVE;
SET GLOBAL innodb_fast_shutdown = 0;
vi /etc/my.cnf
Comment out log-bin=master-bin
service mysql restart
START SLAVE;