There's no need to ever defrag any device or computer running Android or Linux. Never, ever.
It's possible clearing caches as Laura Knotek suggested will help, but it should be done very rarely, only when you have a problem that might be cache-related.
If the phone is still slow after clearing caches, you have other trouble, such as the phone is out of storage space or an app is fouling up operation.
Android since v1.0. Linux since 2001