Why you should be testing your 404 pages web performance
Nice overview of the issue encountered by users if your 404 error page weights too much, with actual data from HTTP Archive.
I would add that looking at 404 errors in your own HTTP server logs (or your CDN ones) will reveal some interesting patterns, like those from Netlify Analytics I've shown in this note.
Some of these patterns are not listed in Matt's article because these are not resources actually linked from pages, "visible" by HTTP Archive's crawler.
They can be malicious attempts to hack your site (
/wp-login.php for WordPress for example), files automatically requested by browsers not used by HTTP Archive's crawler (
apple-touch-icon.png requested only by Safari on iOS for example), etc.