By default, every time you save a post, the earlier version will be kept as a revision. This can be useful in some situations, mostly when you have several editors working on the wordpress site or blog, and there may be problems with the edition. Post revistions allow you to return to any previous saved state of the blog post.
But this feature comes a cost. Yes, it’s free, we mean, at an IT cost: keeping all the earlier revisions will hold space on your database and make some queries slower.
So what can you do to speed up the queries in WordPress? Cut down the number of revisions, or, more radically, turn them off completely.
An example to keep only 2 revisions. Put this line in wp-config.php
define( 'WP_POST_REVISIONS', 3 );
More details about this here, in the WP Codex: http://codex.wordpress.org/Revisions
Ok, and what if I want to delete existing revisions in an easy way?
This plugin will allow you to do so: https://wordpress.org/plugins/better-delete-revision/