Yeah, my 3 computer expert friends say otherwise.
If you are using up to like 50% or more of your hard drive, it tends to start to slow down a tad.
Considering the disk in the hard drive is
spinning constantly, when a file needs to be read or modified, the read/write head could be at any random spot which gives an equal chance for the file to be found.
It's a little different when files are fragmented, because then it has to find the file in the FAT.