It’s a popular WordPress myth that storing multiple post revisions slows down your website. This is not true. WordPress is written smart enough to ignore revisions while rendering the front end and its MySQL queries are written well enough so revisions don’t actually slow them down. However, post revisions take space in your WordPress database. Revisions are practically full copies of your posts, so if you have hundreds of posts this means you have few hundreds revisions stored too.
To sum up, if you need to decrease the size of your WordPress database, removing your post revisions is a great way to do it without sacrificing any actual content. To do this open your wp-config.php file and add the following configuration line to it:
define('WP_POST_REVISIONS', false );
Doing this, however, will only tell your WordPress application to stop storing new post revisions. If you want to delete all the existing ones, it’s a good idea to use the Bulk Delete free plugin.