Recent Windows versions defrag automatically. Mine runs at 02:00 US Central Time the same day every week. You can manually order a defrag, but if the automatic run has been turned on it won't change anything.

Keeping a fair amount of free space on your hard drive is important, since by default Windows won't generally use more than 20% of the free space for temp files or "virtual RAM." You can change the percent used, but that can cause other problems. (Very old systems were limited to the largest single block of contiguous free drive space, making defrag very important to defrag the free space; but newer ones can span free blocks.)

Unless you deliberately change things, the basic free space requirement only applies to your System Drive, since the computer won't use the free space on other drives for swap files and the like, unless you intentionally tell it to (and you can't even do that with the "Basic" versions of newer Windows that many people run).

You can assign free space on other drives for use as temp space in some programs, but it's only easily done in some of them.

If you rely on manual defrags, defrag itself may require around 20% free space on the drive you're defragging if it's badly splattered, but even that space requirement drops if you run it often enough to keep things orderly. Most people shouldn't need to manually run defrag for Vista or later, although it doesn't hurt to check occasionally to be sure the scheduler is running it for you.

The "speed" referred to above is just fast enough so you don't go to sleep and fall out of your chair. It ain't "performance speed." It's just fast enough for the system to keep up with itself - which in some cases it doesn't quite do.

It's not how much is on the drive. It's how much has been changed recently that bogs things down, since the idle-time (background) updates for one change may not be completed before a sugsequent change occurs.

NTFS tries to keep track of more information than it can handle when faced with multiple sequential changes. Give it time, and it will sort it all out, unless you hit it with another flurry of changes before it digests what it's already working on (which I do about daily at present).

John