Is there a maximum IOPS for MySQL asynchronous replication on the slave server?
The immediate fix for the Slave

SET GLOBAL innodb_flush_log_at_trx_commit = 2;


This should speed up the slave a little.

Go to my.cnf on the Slave and add

[mysqld]
innodb_flush_log_at_trx_commit = 2
innodb_flush_method=O_DIRECT


and restart mysqld.

Please read my old post Dynamic change to innodb_flush_log_at_trx_commit for more clarification.