If you write and delete extensively from a storage medium, it gets fragmented. To solve this, you should use a defragmentation tool like Windows’ Defrag.
This makes a lot of sense when using hard drives since the seek time (time to move the heads to the correct location on the disk) is one of the biggest bottlenecks.
So you would think that the effect of fragmentation would be minimal on flash memory storage media like SD or CF cards. This article on Flash Memory Fragmentation proves otherwise. It shows that a fragmented disk 75% slower than a defragmented one and even 87.5% slower than an empty disk.