Receive a 504 and 429 error Too Many Requests

If you receive a 504 or 429 error while working with lazy blocks in the page editor that means that your server is limiting too many AJAX requests.

Block previews are loaded over AJAX, and that is usually the cause. Twenty lazy blocks on a page means twenty requests while the editor loads, and a server with a rate limit answers some of them with a 429 or drops them with a 504.

Pro renders those previews once while the edit screen loads and hands the markup to the editor, so the editor makes no request per block at all. Performance explains how that works and how to switch it off again if a preview goes stale.