[NavinF]

Defrag absolutely helped in the winxp days if your HDD was always >90% full. Every chunk of data written to a full HDD would get spread across the platter, cutting your read and write IOPS to a fraction of what you expect. It was especially noticeable on laptops since their platters spin slower.

[mrich]

For most file systems performance suffers at latest at the 90% full mark, if not before.