It seems that SSD performance decreases over time, since unlike hard disk drives (HDDs), NAND flash memory that make SSD cannot overwrite existing data. This means that you first have to delete the old data before writing new one.
Flash memory is divided into blocks, which is further divided in pages. The minimum write unit is a page, but the smallest erase unit is a block. This means that as time goes on, the SSD will internally fragment the blocks among the different pages, until that it reaches a point where there won’t be available any empty page. Then every time the drive needs to write a block into any of the semi-full pages, it first needs to copy the current blocks from the page to a buffer, then it has to delete the whole page to finally rewrite the old blocks along with the new one. This means that as time goes on the SSD performance degrades more and more, because for every write it has to go through a cycle of read-erase-modify-write. This is known as “write amplification”.