MySQL Replication: Seconds Behind Master keep growing
The solution was to add the following to my.cnf:

innodb_flush_log_at_trx_commit = 2


Our server was slow in I/O and the row above reduced the disk usage.

It is not recommended for bank databases, but recommended to all others.

Phrased differently:  =1 flushes at then end of each transaction, thereby making it ACID-compliant.  =2 flushes to disk every second, thereby being much less I/O-intensive.