MySQL replication using a lot of IO

I'm having big troubles on a database pool I've setup recently.

There is a master, and 4 slaves replicating one DB. All using MyISAM engine.

Thing is, during the replication period (when the slave is not up to date), the IO% showing in iotop is equal to 99% for mysqld.

Besides, I'm having a big latency when connecting a web application to any of these databases (master included).

All these servers are virtual machines running Ubuntu 12.04, having 4vcpu each and 12GB RAM.

If you need more information about the config (system/mysql), please tell me, I'll update my post.

Thing is, I need some hint about where to search, because for now I was not able to identify the problem.

EDIT:

I'm adding the conf files for the master and for a replica :

master configuration file

replica configuration file