Well, sometimes is not much that it's poor code but a lot of data to render. For example, imagine rendering a form with 100 elements. If it takes 0.1 second to render that on a average computer, you can make that 0.01 by rendering it on the server instead.
And then you can cache it. Whats the point of downloading megabytes of javascript, calling 80 different endpoints and then generating what could've been statically saved from the beginning and sent in a single request?
Smart rendering would involve using a mostly static template that's updated with the relatively few dynamic elements as the page is being readied to be sent over the wire. If 90% of your dynamic content is the same, why put that through your app processes at all?
