Laggy slave OK to have only one system user in processlist?

In all the examples of a processlist on a mysql slave I've seen, there are two system users. I have a slave that is very far behind the master. When I do show a processlist, there is only one system user (State: "Waiting for master to send event"), the event scheduler, and a trigger user. Is this a symptom, cause, or nothing to do with excessive lag?

Both servers are 5.6.19 on FreeBSD. Topology: master --> slave --> slave in question. Slave IO and SQL threads show as running. Relay_Log_Space is very large.

show slave status\G
*************************** 1. row ***************************
           Slave_IO_State: Queueing master event to the relay log
              Master_Host: 10.02.00.0
              Master_User: repl
              Master_Port: 3306
            Connect_Retry: 60
          Master_Log_File: master-bin.004634
      Read_Master_Log_Pos: 473351239
           Relay_Log_File: slave-relay-bin.004460
            Relay_Log_Pos: 506886175
    Relay_Master_Log_File: master-bin.004515
         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: 506886012
          Relay_Log_Space: 62863753813
          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: 245653
Master_SSL_Verify_Server_Cert: No
            Last_IO_Errno: 0
            Last_IO_Error:
           Last_SQL_Errno: 0
           Last_SQL_Error:
  Replicate_Ignore_Server_Ids:
         Master_Server_Id: 42
              Master_UUID: a36f5f4b-0904-11e4-a130-b8ca3a622758
         Master_Info_File: /var/lib/mysql/master.info
                SQL_Delay: 0
      SQL_Remaining_Delay: NULL
  Slave_SQL_Running_State: closing tables
       Master_Retry_Count: 86400
              Master_Bind:
  Last_IO_Error_Timestamp:
 Last_SQL_Error_Timestamp:
           Master_SSL_Crl:
       Master_SSL_Crlpath:
       Retrieved_Gtid_Set:
        Executed_Gtid_Set:
            Auto_Position: 0
1 row in set (0.01 sec)


show processlist\G
*************************** 1. row ***************************
 Id: 1
User: event_scheduler
Host: localhost
 db: NULL
Command: Daemon
Time: 74
State: Waiting for next activation
Info: NULL
*************************** 2. row ***************************
 Id: 87720
User: system user
Host:
db: NULL
Command: Connect
Time: 8650
State: Waiting for master to send event
Info: NULL
*************************** 3. row ***************************
 Id: 87721
User: routine_user
Host: localhost
 db: db_name
Command: Connect
Time: 246665
State: closing tables
Info: SELECT
    IFNULL(CAST(LEFT(CONCAT('0', charValue), 10) as decimal(18,9)), 0)
INTO
    converted_