I have deployed my website to a production server... When inspected through firebug i found lot 404 not found errors in my requests.... Mainly from ajax libraries like scriptResource.axd files....
I saw the load time was greater for all the files which was a 404 Not Found...
How expensive is a 404(File Not Found) Error?
Why Timeline is greater for files with status 404 not found.....
I suppose it all depends in what your 404 page does : if it logs a lot of stuff, or does a lot of SQL queries, or is enourmous, it will take time to generate -- like for any other page, actually.
(I've seen a website on which the "error page" was the one that took the most time to generate, as there were a lot of calculations made to try to "suggest" interesting pages to the user)
A 404 request should not be particularly expensive. However, if the 404s are for static content like stylesheets and images, then the browser would normally cache the result. When a 404 is returned this does not happen and the item is requested time and time again. This can have a big effect on page load times.