Why your SSD slows down when it is almost full

An SSD cannot overwrite data in place. To change a block, the controller has to write the new data to a free area first and clean up the old one later, a background job known as garbage collection. When the drive is nearly full, there is little free space to work with, so the controller spends more time shuffling existing data around before it can accept new writes.

The fix is simple: keep roughly 10 to 20 percent of the capacity free, and make sure TRIM is enabled so the operating system tells the drive which blocks are no longer in use. Most modern systems do this by default, but it is worth checking after a clean install or when using an external enclosure.