Extremely slow MyISAM slave updates
You can compare both slaves performance using Percona utilities mentioned below:


pt-heartbeat measures replication lag on a MySQL server. You can use it to update a master or monitor a replica. If possible, MySQL connection options are read from your .my.cnf file.
pt-table-checksum performs an online replication consistency check by executing checksum queries on the master, which produces different results on replicas that are inconsistent with the master.