WordPress drop-in plugins are special plugins that get used when WP runs on your server and modify its behavior. They are called drop-in because you don’t have to activate them. In fact, you cannot activate or deactivate them. They are active when the plugin file exists (is dropped in) in your
One such drop-in plugin is
object-cache.php, which modifies the way WordPress handles transients. You can set it up to use Alternative PHP Accelerator (APC or APCu) or any other caching layer of your PHP server (as opposed to your database), or even use a different table in your database. If you don’t have APCu, it is possible to simulate it.
In fact, the mechanism of simulating APCu, and using the RAM disk of your server (if it is a linux server, that is) is available as a convenient plugin that I have developed, called Easy Object Cache.