Very slow Intel 660p 2 TB M.2 in Icybox enclosure
SSDs use flash cells to store data. At first, only one bit could be stored in each flash cell. As technically got more advanced, more bits could be stored in each cell. This allows SSDs to become cheaper. There's a major disadvantage, though: writes are way slower. When your drive is new, it uses the QLC (4 bits per cell) as SLC (1 bit per cell), which is much faster. As you fill it up, it has to convert SLC cells into QLC cell, and soon you only have QLC cells. This is only made worse by the fact that you're not running TRIM commands. TRIM tells the drive which space is not in use anymore, which means whenever you write to an empty space, you don't have to erase the cell before you write to it. That's why you don't see any performance increases after deleting files, it still things the drive is full, and thus won't use the "SLC cache" feature.

Relevant video: https://m.youtube.com/watch?v=OffzVc7ZB-o