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 loaded using AJAX and this may be the reason. If you add too many blocks (for example 20 blocks) to the page, then the initial editor load will fire 20 AJAX requests. We have a preloading feature in the Pro plugin, which may solve this problem with initial AJAX requests.