The flash memory used on today's SSDs is comprised of cells that usually contain 4KB pages arranged in blocks of 512KB. A memory cell has to be empty before data can be written on it, even if it's only partially filled. So when it already contains some data, the entire block must be re-written, combining the data with new information going to the cell and overall adding to the time an SSD needs to complete write operations.
Luckily, most manufacturers are already attacking performance degradation issues head on via firmware updates and a process called “garbage collecting,” which basically kicks in whenever an SSD is idle to seek out and clean flash memory cells that hold old or redundant data.
OCZ and controller manufacturer Indilinx, for example, have collaborated on a new firmware for OCZ's entire Vertex line. While still in beta, early tests of this firmware from PC Perspective and Hot Hardware show that just a few minutes of idle are able to noticeably improve performance – regaining like-new levels in about an hour. According to PC Per, upcoming builds should also support the upcoming TRIM command for Windows 7, further reducing degradation and wear level of the drives.