Why would one disk defragmenting program take so much more time than another?

Defragmenting

If you've ever had your newspaper get spread out all over your yard in the wind and then had to go out, pick it all up, and put it back together in order, you're familiar with defragmenting.

Over time, the same thing happens to the files on your hard disk. Individual pieces of the files get spread out all over the surface of the disk so that when the file is actually used, the hard disk must physically move the read/write head all over the place to access the individual pieces.

All of that movement makes file access slower.†

Kind of like walking around your yard to read the paper, but leaving all of the pages where they landed.

Defragmenting moves all of the pieces of files together in order, so that they're easier and quicker for the disk to access when needed.

That's somewhat like walking around your yard once to collect the pieces of newspaper together and putting them back together in order. Now reading the paper is simply a matter of starting at the beginning and turning pages.

Quick Defragmenting

Typically, a defragger that runs quickly focuses exclusively on exactly that: collecting files and putting them in order.

And it might not complete the job.

What you might find after running a defragmenting program that completes quickly is that there are still some files left in a fragmented state. Somewhat like asking your teenager to pick up the newspaper - it might do the parts that are easy and pick them up quickly, but it leaves a few "less important" sections where they were.

The result is definitely an improvement, particularly if the commonly used files are defragmented, but it's not complete.

Thorough Defragmenting

A more time-consuming defragmenter might well be much more thorough, making sure that each and every file that can be defragmented is completely defragmented.

Depending on how much free space there is on your drive, that can actually take a fair amount of additional time.

On top of that, while not technically defragmenting, some tools perform an additional optimization and move all of the defragmented files on the disk to the same area of the disk.

That's somewhat like bringing in all of the newspapers into the kitchen after collecting and reconstructing them in order. Not only are the newspapers easier to read, but they're all collected in the same place in the house and thus easier to locate when needed.

Collecting all of the files together like that can take additional time.

How much is enough?

While a perfectly defragmented disk sounds desirable, you're actually not likely to notice much of a speed improvement compared to a disk where only most of the files have been defragmented.

As a result, it just may not be worth it to perform regular, lengthy defragmenting. A simple periodic defrag might be more than enough to get you the speed benefit that you're looking for.

The one exception that I find myself making to this is after installing Windows. There's so much file activity during the installation process to files that are a) never written to again and b) read frequently as part of Windows' own operation that a thorough defragging can make sense to put those files in their most optimal positions.

After that, periodic and quick is good enough.

The tools to use

Windows' own built-in defragmenting tool is easy enough for periodic defragging. In fact, if you're running Windows 7, you don't need to do a thing: Windows 7 schedules a weekly defrag of all of its volumes by default. You can set up your own scheduled defrag in previous versions.

There are many, many third-party defragmenting tools available if you want a more thorough scan. Of late, I've been using Piriform's Defraggler for the task.

As an alternative to a third-party tool, you can also run the Windows tool repeatedly until it shows no or a low number of fragmented files.

It's not something that I worry about

I'll repeat what I said above: a lot of extra effort in defragmenting your hard disk isn't going to result in a noticeable speed increase.

Consider a thorough defrag after you set up your machine or maybe every year or two thereafter. Then, set up an automated weekly defrag and move on to other things.

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† This does not apply to solid state drives, such as SSDs and Flash drives. Because there is no physical disk head to move, defragmenting has no effect on performance of these at all, and as flash memory wears out, defragging solid state media can seriously shorten its lifespan.