I've seen a few random pages mention using empty gif images to somehow increase performance. I've also found the nginx has a module for just this purpose. What I can't figure out, is exactly how serving this small file is supposed to boost performance or perceived responsiveness from a web server. Can anyone help me understand the benefits?
1x1 gif files are used by some websites to set spacing between elements (particularly on older websites, made when browsers' interpretations of HTML/CSS were more divergent cough IE cough
They are also used more often today as a request target for "tracking pixels", which are used as a tool for gathering usage stats, etc., especially for email/marketing campaigns.
The reason you'd provide a special module for this file is that (a) it's requested often, and (b) it returns the same thing every time, so you don't want to have to go to disk for the file if you can avoid it.
I'm not sure about increasing performance, but 1x1 pixel images are commonly used in tracking. Commonly known as a web bug (or beacon), this image causes the browser to make an HTTP request to the server from which the image originates. This request can then be examined for tracking information (e.g. IP, browser, referrer, etc).
From Optimizing Design: One-pixel GIFs (January 1, 2000):
In the early days of web design, HTML limitations inspired designers to find all sorts of workarounds in order to have control over the layout of pages. One of the most significant of these workarounds was the single pixel GIF trick, using a single pixel GIF and modifying it with the height, width, hspace, and vspace attributes of the IMG tag.
The GIF itself is tiny and requires virtually no download time. And, depending on whether the single pixel is transparent or not, the result can be used as a spacer, or as a horizontal or vertical rule.
Including 1x1 images doesn't increase performance, it decreases it - it's an extra HTTP request, a small amount of extra bandwidth, etc.
That module is simply intended to make the decrease in performance smaller (by serving the image more efficiently), as the 1x1 pixel technique for positioning elements on the page can be useful.
They are also used for image spriting; a small transparent image is used as the src attribute of an image tag, and the real image is a CSS background image. The performance advantage comes from the CSS background being only part of a single image which combines many images from the page, reducing the number of overall requests.
See for example:
All great answers above. There is another use for the module.
When you want to block ads from being displayed in your network, if you simply deny the URLs requested the browser load times suffer (or you get a block message displayed on the page). The solution then is to redirect browser traffic to an internal site serving a blank image (rather than deny the request URL).
Some tricks that can be used to do the redirect are Dnsmasq and Squid rewrite rules.