Complex sentence: The previous Amiga filesystem (known originally simply as "DOS" or AmigaDOS) upon the release of FFS became known as Amiga Old File System (OFS) was too slow to keep up with hard drives.
Split into simpler sentences: The previous Amiga filesystem (known originally simply as "DOS" or AmigaDOS) upon the release of FFS became known as Amiga Old File System (OFS). OFS was too slow to keep up with hard drives.