Examine the modified output:Seconds_Behind_ Master value is steadily growing.

The master is most probably too busy to transmit data and the slave needs to wait for more data.

One or more large tables do not have primary keys.

This value shows only I/O latency and is not indicative of the size of the transaction queue.

The master is producing a large volume of events in parallel but the slave is processing them serially.

The parallel slave threads are experiencing lock contention.