Slow loading server rendered would also be a slow loading API. At least with a slow server rendered app I as a user (not a developer) knows something is happeni

Slow loading server rendered would also be a slow loading API. At least with a slow server rendered app I as a user (not a developer) knows something is happening. A fully formed client with a slow backend can give the wrong impression that the app is working (unless the developer reimplements "loading" feedback that the browser implements natively)