Cookies really don't take up all that much room on your hard drive -- maybe a few megabytes at the most. Hardly enough to warrant "freeing up space."
Is your hard drive close to being full? If so, I would advise a more aggressive approach to freeing up space. Hard drives don't run well today if they're kept at close to capacity... you should leave, ideally, about 15% to 20% of your hard drive free (for example, for a 20GB drive, you should leave 4GB to 5GB free, all the time).
It very well could be that your computer is running slow if your hard drive is too full, as described above.