When excessive requests are made to assets or pages that return a 404 error, our servers will return an HTTP 428 Precondition Required response code.
As 404 pages are not cached, multiple requests that 404 in a short period of time can affect site performance and speed. By rate-limiting excessive requests, sites are less likely to be impacted by these requests.
If you are experiencing a high number of 404 requests you may want to enable our lightweight 404 page option, which bypasses WordPress to allow the server to handle 404s.