Implementing Read-Replica (slave) database connections
Craft 3

You can see an example of how to do read/write splitting in Craft 3 here: https://docs.craftcms.com/v3/config/app.html#database-component

Craft 2

Definitely not current possibly natively in Craft since Yii's CDbConnection (which is what Craft uses for database connections) has no concept of master/slave connections.

In theory, something like the Yii DB splitting extension that @RitterKnight mentioned is a start in the right direction, but even then, you'd have to do some pretty significant core hacking to utilize it in Craft 2.

Should be much more feasible in Craft 3 since the ability to swap out core components like that is planned.