It's amazing, and concerning, how bloated and cluttered a WordPress database can become.
I set about cleaning mine up yesterday using a couple of different plugins and found a plethora of redundant or orphaned entries which needed removing. Not only that but I reduced it from 34 tables down to 12!
Admittedly, my database is almost 12 years old but that's an awful lot of junk not automatically removed by WordPress itself or by plugins when they are uninstalled.
Does the site work faster now? I dont know, maybe, or that could just be a placebo effect. Regardless, the database is nice and trim and that can only be a good thing.