Yeah it would be great to see some numbers from anyone who might have them. I'd put money on SPAs being slower, inside the bell curve than, than the average traditional page load app.
JSON may or may not be smaller or faster: if you have to load data you don’t need or, worse, chase links it’ll be worse. GraphQL may help but that’s bringing it closer to server-side performance, not exceeding it.
Things which aren’t possible otherwise are the best argument for SPAs, but another approach is progressive enhancement: you can load quickly and then load features as needed rather than locking in a big up-front cost if all you need are real-time updates or push notifications. There’s a spectrum of capabilities here and there won’t be a single right answer for every project.