I can think of two possible reasons. 1) NTFS performance drops when more than 2/3 of the disk/partition is in use. That fits with the numbers you quote. Solution: get a bigger harddisk. 2) Fragmentation. The free space you have could be one big chunk, or it could be lots of little chunks. The latter is called fragmentation. It can considerably slow down your disk system. Solution: use a defragmentation tool on a regular basis. The defragmentation tool that comes with Windows does the job, though I prefer Diskeeper (better defragmentation and nice scheduling options).

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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