The big issue (as the article says) is that TLC/QLC drives have a faster dynamic SLC cache.

Skip this if you know what the terms mean:

• SLC (Single Level Cell) means per cell there’s one bit stored
• MLC (Multi Level Cell): one cell stores 2 bit
• TLC (Triple Level Cell): 3 bit
• QLC (Quad Level Cell): 4 bit

So the more bit per cell, the more storage the drive has, but the slower it is.

Because of that SSD controllers use part of the free space on a TLC/QLC drive in SLC mode as a fast cache, trading unused storage space for more speed.

The fuller the drive gets the smaller the cache will be, and on an almost completely full drive there is no cache.