One possible reason why an SSD might slow down during copying is that it becomes busy with other operations, such as garbage collection or wear leveling. These processes are designed to help maintain the performance and lifespan of the SSD by redistributing data across the drive and freeing up space. However, they can also take up valuable resources that would otherwise be used for data transfer.
Another factor that can affect the speed of an SSD during copying is the way that the data is being written to the drive. SSDs use a technology called NAND flash memory, which allows data to be written in blocks. If the data being copied is not aligned with these blocks, it can cause the SSD to slow down or even pause during the transfer.