Can the WordPress rest API be used by thousands of public users at the same time
Public users can use the REST API yes, though their access will be more restricted, e.g. you can see my sites rest API at tomjn.com/wp-json, but good luck trying to create a post without a login 🙂
or is it limited for only developers to use?
As I mentioned, some endpoints require authenticated access, e.g. modifying and creating posts. Adding your own endpoints you can configure this
I want to use the API to serve content to thousands of users, but am finding that currently it is slow in pulling data from the server
Yes, there are websites powered entirely by the REST API with thousands of concurrent users, but don’t expect a shared hosting setup to be able to handle it.
REST API calls still take time and system resources, and if you can’t serve the front page to 1000 users at the same time, the REST API won’t be much different.
So the real answer depends on how well your site is built and the hardware object cache, and caching mechanisms backing it up
I.e. images, etc.
REST APIs return JSON data, sometimes XML. They don’t return images. Your JSON response might contain a HTML fragment, but any images referenced are just referenced. The same way PHP sending an image tag is just referencing them.
If you’re hoping to send entire HTML pages via the REST API, as if it was the frontend, then you’ve got the wrong end of the stick. The REST API is not intended for humans. JS calls to your site, remote requests, tools etc, but not real people visiting.
Sites that use the REST API will send over a page, and that page will then make calls in JS to the API to fill in the gaps.