It's true, but it's more true for some file systems than others. When you write a file larger than the contiguous available space after its starting point, a f

It's true, but it's more true for some file systems than others. When you write a file larger than the contiguous available space after its starting point, a file system must break the file into "extents" (chunks). The less space available, the smaller the extents tend to be, and the more fragmentation you will impose for continued writes. It's just math. Different file systems have wildly different strategies and data structures behind this process, however. Some drop to their knees over 92-93%. Some can write to the last byte with reasonable efficiency—but it'll never be as fast as when it was empty. Copy-on-write systems like ZFS tend to do poorly under near-full conditions.