... The graduated slowdown of SSD (which according to the article, applies to all SSD when they are filled up, not just Intel's); The stuttering issue caused by certain controllers; And more...
This is due to long erase time of NAND Flash used in SSDs, MLC or SLC. A blank storage cell is consider one when in it's initial state. Zero data is written into the cell by placing electrons into the so called floating gate. So cells with one can be written to zero but cell already written to zero cannot be reprogrammed to one again. The programmed zero cells can only be restore to one state again by an erase operation. This is removing electrons from the floating gates and usually done in blocks with multiple rows.
When the SSD is filled, the internal controller start to reclaim available space by erase. So performance will slow eventually. More advanced controllers will do reclaim/erase in the background so there will be available space when write occurs.
While large manufactures with long history such as intel, Samsung, M-Systems (acquired by Sandisk) incorporated these technologies; it is not clear smaller manufacturers are able to acquired controller with background erase/reclaim function.
The faster write time in newer SSD's likely come from data being written in parallel, to two or four chips at the same time. But it won't improve erase time much.
Large data write is fast because all data is written at the same time to one or multiple rows in the chip memory array. Small data write is slow because the data size is smaller than the row size. The controller then do a read row, modify, write row operation; read the entire row including existing data, append new data to the end of existing data, write the entire row with new data back to the memory array again. Most problematic is the FAT area where lots of small read and write are occurring. This can be improved by using NOR flash for FAT. But this adds to cost.
HDD is not affected by this erase problem so read and write time is almost equal.