Over time, most solid state drives slow down with use as their internal blocks become severely fragmented (not conventional file-system fragmentation).
The slowdown can sometimes have an impact on read speed, but write speed is usually most impaired, though it all can depend on usage and brand and firmware version. Read more about this in Reconditioning a Solid State Drive and my review of the Crucial 128GB SSD.
The DiskTester recondition command works fastest and achieves the best results when a significant amount of free space is available. If you’re using a Windows PC, then there might be a toolkit for your brand of SSD available from the manufacturer; as far as I know, no such toolkit exists for Mac OS X, though Macs can be booted into Windows with some effort.
A sea-change in SSD behavior?
I have high hopes for the new OWC enterprise-class solid state drive as being perhaps the first SSD on the market to sidestep this lingering issue with SSDs, as well as offering far better longevity and data safety than other models on the market today.
My review of the OWC solid state drive will be published sometime later this week.