But if there's an API, then the overall load is the same, no?
Or to put it another way, naively, having api.example.com and realpeople.example.com separated out into separate sandboxes seems reasonable, but due to the aforementioned problem, its not. But then it also turns out to be the wrong axis for this anyway, and you need your monitoring to work for you.
No, the load isn't the same because the web page might be a multi-megabyte monster piece of badly-coded HTML that returns only say 10 out of 1,000,000 results and needs to be paged through, where the API might return all the million results in a nice JSON chunk.
Or to put it another way, naively, having api.example.com and realpeople.example.com separated out into separate sandboxes seems reasonable, but due to the aforementioned problem, its not. But then it also turns out to be the wrong axis for this anyway, and you need your monitoring to work for you.