Just noticed while doing some spring cleaning in the site's database that a lot of our most used tables had 70% + fragmentation on their indexes.
Made sure to rebuild and reorganize all of them, and they're now sitting around 0-.1% fragmented.
Our DB is in SQL Server, but should apply to you MYSQL guys as well.
Pretty interesting that I believe our site is pretty optimized, but you can still stumble upon something like this
