I'm sorry, but this article just doesn't make sense in several places.
The thing is, all computers slow down....Stuff can be pictures, music, videos, email, drivers, spam, and whatever else you have obtained, knowingly or unknowingly.
This is quite clearly untrue. Storing 'stuff' on your computer will in no way slow it down, so long as you don't approach the limit of the hard disk (and thus possibly impact paging memory to disk). This is a common misconception among users who don't know the difference between RAM and hard disk space, and this author is encouraging the confusion by repeating this falsehood several times in the article.
1.Throw Away Data You No Longer Need and Defragment Your Drive
What difference will throwing some documents away make? It may clear up disk space, but it most certainly won't increase performance. Defragmentation is no longer required in Panther, but the author makes no mention of that. As to 'two to four times a year' - pheh.
2. As mentioned in the previous section, when you reduce clutter, you improve the performance of your system.
Once again, this is repeated - would the author care to explain why this is the case? Removing applications will NOT improve your system's performance. If you don't run as many applications all at the same time, you might impact performance.
6. Get Rid of Font Gunk
This is very dangerous advice for people who aren't sure what they're doing - which I assume is the target audience. Most of those people are unlikely to have a lot of fonts on their system anyway. While removing fonts can speed up various applications, it can also break the system in subtle ways. A list of essential fonts would be good, and I'd leave out the advice to remove 'bold, italic versions'. This advice is a recipe for disaster and won't significantly impact system performance.
I won't go on, but this article has some serious flaws, in particular the vague summary which implies that anything copied to a computer's hard disk will slow it down. The best advice they could give anyone who wants more performance under OS X is to buy more RAM; as much as they can.