I think computers are mainly slow because of slow antivirus like AVG 2011, and bloated programs like Internet Explorer 8, toolbars, poorly made browser plug-ins, etc being opened and running in the background of your PC which makes your PC feel heavy or slow because of them running in your RAM/memory which makes Windows slow, and not files which are just being stored on your PC hard drive not being opened.
As long as you defrag your hard drive once a month or when ever it is 20% fragmented according to your disk defrag analyze report, and you have 20% or more of free space for disk defrag to run properly. There would not be a performance increase or decrease by having Terabytes, or 100s of Gigabytes of free space since it is the computers' Operating system, software being run, Processor, RAM, and video card which usually make your computer faster or slower.
But, if you have less then 20% of free space you may notice a performance decrease because your hard drive can not properly defrag the disk since it can't move your most used files to the fastest part of your hard drive, and the least used files to the slowest section of your hard drive.
I run Windows and Linux on a drive which has 20% free space, and on drives with over 70% free space, and did not notice any performance increase or decrease.