You are both right and wrong. :)
If a web-page contains absolutely no animations, you will not notice any speed difference.
The fancybox creates a transparent 'div', and everything under it has to go through graphics filters, which means a slow down.
Try opening a fancybox-page on a slow machine, which has some kind of animation on the page below and see the difference.
Animation can be anything: text that changes, tickers, animated gifs, flash, movies, javascript that changes colors, etc.
Also when resizing your windoes, you will notice a great slowdown if a fancybox is open.

I will work on supporting legitimate uses, when I have a solid idea on how to do just that.

The transparency should't need to go through filters unless the browser is IE6. Transparency is built into the modern browser. And as a senior level web developer, Fancybox, and most Lightbox effects like it, are usually not the problem. Video and complex Flash animations are usually the problem. The extension may be handy for some, but you are using a sledge hammer to make fine adjustments to your web browsing experience. If that is what you want—go for it.

When you play a flash-movie, it uses for instance 60% of the total CPU time (on a good day with no wind and no rain).
Put a 50% transparent DIV over it, now it uses nearly 100% (and often chops).
This is because the DIV _is_ a filter. Like sunglasses are filters.
The calculations for each pixel suddenly takes a _lot_ of extra time.
The numbers mentioned are based upon a 2x2.5GHz machine.
Today, you might be surprised to know that there are still people surfing the Web using 700MHz iMac G4 models. But know that there are also many people who still use G3 machines below 400MHz (because that's all they have; they're also on slow internet connections as well). Imagine how the Web would be for those people, if they could not turn off images, Flash, Java and Javascript. Noone should base the functionality of a Web-site on JavaScript without having a backup for showing the contents correctly without it.
In other words: JavaScript should be used only for makng a Web-page faster (such as making on-the-fly calculations); HTML is the content and CSS is how it should look. See w3.org for more info.

The div is using css opacity NOT a filter. The css rendering is built into the browser, so it it does not add a lot of CPU processing time. If you are so worried about supporting older and slower computers, use Lynx. I suspect the"numbers" you are talking about show that you have other issues with your test machine.

You cannot stop progress. Web 2.0 is here now. The Lightbox effects make for easier navigation and presentation of a lot of web content and that is a good thing. If users machines are so old and slow as to have problems displaying simple content, your extension will only make things worse, and better yet, your extension won't work on their systems anyway, so you may need to rethink your position.

This is not a necessary discussion.

I'm glad it's usable. As it's not nagware, it shouldn't stop working, unless some webmaster decides to make a workaround on his own website. (I also recommend using FlashToHTML5 or YouTube5; in case you sometimes watch YouTube videos. I'm not the author of those, but I use them to get my CPU usage down. ;)

thanks, packman. i am using youtube5 in fact.