Is it better to have a backend service sending json and using a clientside framework such as AngularJS (or) have a full webapp in the backend serving HTML with minimal javascript?

While choosing between client side rendering and server side rendering you need to understand their pros and cons. Since you are asking for performance concerns, by choosing client side rendering you don't have the control over performance. That is because the performance depends upon the client device and browser capabilities. But for the server side rendering you have the full control over the server and you can scale up depending upon your needs.   We are using client side rendering for our http://www.chorusplay.com mobile web app. ours is a single page application mainly for mobile devices, we use JQuery mobile to render HTML from JSON response   Problems we faced with client side rendering.   Device Performance: Most of our users have medium-end smartphones and rendering in those phones were slow.   Mobile browser fragmentation: Unlike desktop browsers, in mobile there are lot of different manufactures and lot of different browser implementations causes testing and debugging nightmare   Cross domain issues: Since we are consuming data via APIs, we faced lot of Cross domain issues.  Lot of mobile browsers couldn’t handle Cross Domain Requests. We had to do some hacks to solve this.   Content visibility: Content is not visible to search engines like Google. This will affect SEO and organic traffic. (Google’s https://developers.google.com/webmasters/ajax-crawling/docs/specification for solving this.)   Hybrid rendering There are some new frameworks like http://nerds.airbnb.com/weve-launched-our-first-nodejs-app-to-product/ and http://artsy.github.io/blog/2013/11/30/rendering-on-the-server-and-client-in-node-dot-js/ which leverages both client side and server side rendering. They are not matured enough but take a look. Do your homework well before choosing client side or server side. Both have its own cons and pros.  Choose one which is suitable for your use.

Browsers today are fast and powerful enough to handle heavy applications. In a typical webapp, the server should handle tasks related only to database manipulation and authentication, the rest can be very efficiently taken care of by the client side using AngularJS or any other  javascript framework.