mysql replication delay very serious
I wrote about something similar on MySQL replication: most important config parameters for performance on slave server?
Basically you should focus on the follow variables on slave (see the other post for more information):

innodb_buffer_pool_size
innodb_log_file_size
innodb_flush_log_at_trx_commit
innodb_doublewrite
innodb_flush_method
innodb_max_dirty_pages_pct
sync_binlog
sync_master_info
sync_relay_log
sync_relay_log_info

One thing to have in mind is that the slave have just a single thread to process all writes from master, let's say you master are able to handle 300 connections simultaneously, it's 300 threads writing at the same time (respecting locking issues, disk, ...)  but on the slave you have just a single one to deal with all this transactions.
Suggestion:
If you use MySQL 5.6 and have your data partitioned per database, you can take advantage of slave-parallel-workers