Hard disks have "zones." Here's some articles about it. http://www.google.com/search?hl=en&q=HARD+DISK+ZONES&btnG=Google+Search
As you fill the drive, each zone is progressively slower. I won't rewrite the following. The outermost track has many more sectors per track so it's the best performing zone. As you move in (filling the drive) the count drops and performance falls are almost half according to some benchmarks.
Why have such a full drive?
I have a 3 yr old Dell Inspirion 9300 with a 51gb hard drive. I only have 7gb free space left. My question - what % of free space is needed for the laptop to perform well?