WordPress is not a framework first, it is a php CMS first, framework second. I only code using wordpress to extend its functionality as a content management system, adding a much better remote interface in this case. Having asynchronous tasks and only updating changes is very efficient but has its drawbacks especially with wordpress.
My plugin has to keep secrets secret, this means it has to be the in between from client and remote server, adding to the delay. There is no way around this to avoid unnecessary secret info available on the client machine while using a secure connection method to the remote server. In simple terms it works like this (load order page) -> Client requests data via AJAX -> wordpress plugin -> remote server -> wordpress plugin -> client . Quite a chain compared to cutting out the middle man and going straight to the remote server with less security, but rest assured its still faster than refreshing the page in wordpress admin on the remote website. The changes also workout a lot smoother too with only a portion of the page changing with new data of what you’re expecting to see, while also being a lot more secure and only giving the client the data and code they need, not everything from the server that they dont need that could be exploited later on. It adds more steps and work but you should always be thinking about security when it comes to e-commerce, as sometimes bypassing a step might not be the best way in security as the first part of loading wordpress requires a user with sufficient privileges logged in, and should the user not be authorised you do not want to provide them with irrelevant secure data for something else either.
However this chain and delay triggers benchmarks with the turtle sign appearing saying server response is too high, but not having to reload the entire page is a much bigger saver in all aspects even time. This can be significantly reduced with non framework php specific code to address each particular website with their database directly without going through wordpress (no modularity and more configuration when adding to any other website), but it can significantly reduce the response so its not without its cure, if only wordpress plugin developers decided to make their code fast and also good, response would be a lot less of a problem as everytime wordpress is loaded, it has to load its plugins as well and most plugins are not made with speed in mind, only to get the job done. Even the theme has to be loaded as well which shouldnt be needed for an AJAX request. WordPress should set its ajax requests to load only the plugin involved and wordpress and nothing else unless specified for other plugins and themes.
In summary chaining requests like AJAX + REST is still worth it despite the delay, as most wordpress sites are very heavy, sporting heavy plugins that are poorly developed (frameworks in frameworks), heavy themes and page builders (these are a nightmare in page loads and optimisation). Being able to selectively load wordpress or only code related to the query is a lot lot better but wordpress doesnt have this feature only hooks for different stages of loading. The resource savings though is still good despite benchmark scores and warnings, as this is one of the things that often people will tell you not to do but is actually a good alternative than the normal methods available.