When the SSD is brand new from the factory or has been secure erased, all the cells contain no data in them. All the drive has to do is write data to them to get the job done. Once all the cells have been filled even once, what the drive will have to do in order to make a new write is to delete the previous contents of the cell first, and only then make that write operation. Naturally this takes about double the time than writing to a blank cell.

When you fill up the drive with IOMeter test file, all the cells become "filled" and even after you erase the test file, they are still filled because the drive does NOT know which cells contain valid user data and which do not. So it is not about how much space you are currently using, its more about how much space you've used so far overall.

The above applies to most of the performance SSDs...

I have no idea why Vertex drives are seeing a drop in the reads... That logically should not happen and doesn't happen to all the X-25 drives. The degradation should only apply to the writes.

A possible solution to performance degradation is to make a program that would be aware of the internal wear leveling algorithms of the SSD in question and would be able to tell the drive which cells do not contain valid user data so it could go ahead and do the erasing whenever you want it to and not right before doing writing operations. I believe Intel is working on something like this, but it seems like they aren't having too much luck. Programs like this would be SSD specific for obvious reasons... There won't be one program that will support all SSDs.