When you first install WordPress, your database starts off so clean and tiny. But as you start “living” in your site, your database starts picking up all kinds of junk. It’s like dust in your house. You didn’t do anything to “cause” the dust – but it still accumulates and makes everything look dirty.
Posts revisions, transients, table overhead, spam comments, and more do that to your database. Each individual database entry isn’t a big deal – but combined (like those specks of dust), they can make your site a big sluggish.
Thankfully, they’re easy to clean up. All you need to do is run a plugin like WP-Sweep. Nothing to configure – just install, head to Tools.
Sweep and clean up your various tables:
You can also Sweep All at the bottom of the page – though I don’t recommend it because you might wipe out your drafts. It’s usually better to go through each category individually to ensure you don’t remove important information.