We have been noticing a lot of 404 errors being thrown in our Coldfusion CFIDE server monitor, and it took us a while to find out that things like missing favicons are causing these errors.
We use a custom 404 template page, which contains some logic to it (more than just basic HTML). So, whenever a 404 for a favicon occurs, these pages are generated and returned to the user.
Since these favicons are requested by default on many browsers (if there is no in the header that specifies one, it looks in the site root or something like that), it's throwing massive amounts of 404s on our server, which costs processing time as well as bandwidth for delivery. Our server runs fine most of the time, but when it does get some heavy usage, we can sometimes have major performance problems.
I know that this is a performance issue, but would it be enough of one to warrant trying to fix this? If so, is there a way with the Coldfusion server (or our underlying Windows Server 2003 running IIS) to filter what files actually throw a CF 404 error? Ideally, for files like these favicons, CSS, and Javascript (since a visitor never really "sees" the output of these), we would simply want to return an HTTP 404 response with no content, as it is unneeded...
Yes, missing favicons do affect performance, as does any missing content when you have a custom 404 page (esp. if the 404 page is handled by a content management system). This is because every time the file (image, video, page, etc) is requested by the browser, it causes load on your server. Let's say the favicon is missing on every page of your website. If your 404 page is part of your content management system, this doubles the load on your server (basically requesting 2 pages every time instead of 1). If your 404 page is different, but still has logic, this increases the load, but only as much as the file requires (less logic = less load, and vice-versa).
I would suggest fixing this issue, but not necessarily by killing custom 404 pages for certain extensions. In my opinion, it would be better (for you, and your visitors) if you simply added a favicon file to all of your sites. Not only would this solve your 404 issue, but it would help your visitors to recognize your website quicker when bookmarking pages, or adding the site as an App Tab (which would apply, even if your site isn't available to the public, as backend sites are a great use case for App Tabs). Aside from your server's performance, having to download the 404 page causes for network performance as well, both on your server's end as well as the end-user. The 404 page may also not be cached, and even if it is, probably not for as long as an existing favicon would be, which causes the request to happen far more often than it would if you simply created a favicon.
If you don't want to take the time (or don't have a need) to do advanced branding (such as creating a custom logo for the favicon), a basic image with a letter on it (e.g. "K") will do. Favicons are extremely useful for the public, any staff, and even yourself, so I would say it's definitely worth your time to at least do a basic favicon.