The whole point is to make the experience better for the user: When going to each new page on the site only involves fetching a bit of JSON and not an entire HTML page including header, footer, JS, CSS, etc., that makes the user experience faster. Add to that the fact that since the front page HTML now no longer contains any dynamic elements (you would get those dynamic elements via JS), you can put your front page HTML (the one HTML page there is) on a CDN and get faster load times.
As for speed: Sure there are exceptions when developers go crazy with tons of heavy JS, but that doesn't need to be the case at all.
As for speed: Sure there are exceptions when developers go crazy with tons of heavy JS, but that doesn't need to be the case at all.