That should be: Over 100MB, and sometimes over 200MB?

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.

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.