Is there any reason the speed should degrade over time? After all it is using electronic memory.
Flash memory need to be erased before it can be rewritten. Since a virgin SSD is completely
erased, most write operations can start immediately. When it fills up, chances that a block
that needs to be erased is hit will grow. This is causing the slowdown. There are several ways
to avoid this: A dedicated "TRIM" tool, that is run manually by the user (file system specific,
mostly Windows-only). A file system, the emits TRIM SCSI commands to tell the drive which
blocks are no longer in use so that it can erase these blocks when it is not doing any other
useful stuff (I think Windows 7 does this). Hardware based "garbage collection" solutions, that
combine block erasing with remapping. The latter is needed anyway in order to avoid premature
wearing out of flash cells in case the same block is written over and over again.