Mysql Replication Lag is increasing continuously

Hi Team,
Could anyone please help me to come out of this situation. When i actually change the
pt-online-schema-change -u root --ask-pass --alter “MODIFY COLUMN id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT” on production changes reflected. But it has a slave where i see the relication delay is continuously increasing. unfortunately i dont see any long running query in slave. The queries are changing. Master_Log_File: mysql-bin.001797 Read_Master_Log_Pos: 289879640 Relay_Log_File: mysqld-relay-bin.005384 Relay_Log_Pos: 217855411 Relay_Master_Log_File: mysql-bin.001788 Slave_IO_Running: Yes Slave_SQL_Running: Yes Seconds_Behind_Master: 66980 The table we are updating is around 30G.Please ask if for any queries. Thanks for your time in advance.