Why does my SSD slow down when it is nearly full?

Solid state drives need free blocks to write efficiently, and when they run short the write process becomes substantially more expensive. This is not gradual wear; it is a change in how the drive operates.

An SSD cannot overwrite data in place. It writes to erased blocks, and erasing happens at a much larger granularity than writing. With plenty of free space the drive always has pre-erased blocks ready, so writes are fast. When space is scarce, the drive must first consolidate partially used blocks, erase them, and then write, turning one operation into several. This is called write amplification and it can slow writes dramatically.

Drives reserve some hidden capacity for exactly this purpose, but consumer models reserve relatively little, so user free space matters.

Many drives also use a portion of their storage in a faster mode as a write cache, falling back to a slower mode when that region is full. On a nearly full drive that cache region shrinks or disappears, which is why large file transfers can start fast and then collapse to a fraction of the speed.

The practical guidance is to keep roughly ten to twenty percent free. That is a rule of thumb rather than a precise threshold, and the penalty grows sharply as you approach completely full rather than appearing suddenly.

One related mechanism is worth knowing. Deleted files are not immediately erased at the hardware level; the operating system signals which blocks are no longer needed so the drive can reclaim them in the background. That signalling is enabled by default on modern systems, but on an unusual configuration or an external enclosure it may not be passed through, in which case performance degrades over time even with free space available.