Do SSD drives slowdown when near full?

[James]

This is not true.

A spinning HDD sees performance degradation for every additional bit written, because of the distance you have to travel across the platters.

This is the reason that in a lot of IT environments before the advent of SSD's and relatively cheap RAM, "short stroking" was a common practice. Unlike CD's, HDD's write from the outside edge of the platter inwards. This means two things. 1) as you move towards the middle of the platter, each revolution of the platter passes fewer sectors beneath the r/w head. 2) as ImaNihilist said, as you fill up the disk, random reads and writes take longer as well, as you can have wild swings of the r/w head back and forth across the full surface of the platter.

Getting back to "short stroking" (insert your favorite joke here). This was the practice of using an extremely small percentage of the HDD's full capacity to ensure that all of your data was being written to and read from the fastest sections of the disk, as well as minimizing the distance that the r/w head had to travel to do so. Entire banks of hdd's would only be formatted to use 10-20% of the listed capacity to ensure maximum speed of the storage backend. Extremely wasteful, but at the time it was the only way to really increase performance of something like a database that was too big to fit in RAM.