But I doubt the bottleneck is JS code. The problem is that web sites are not optimized (some frontend developers think that writing a CSS stylesheet for narrow screen is enough) and they include a lot of resources (including trackers, advertisement, spying social network buttons I never click). Some of the widgets create an iframe (which is like a separate tab in your browser) and load a separate copy of jQuery there, make AJAX requests etc. And even worse, some advertisement networks can create nested iframes 2 or 3 levels deep (for example when a network doesn't had own ads, they can put Google Adwords block). So when you load a page with 10 iframes it loads the CPU as 10 separate tabs.
Decoding images is not free too, especially if it is thousand pixel wide heavily compressed JPEG or PNG image.
The real optimization would be cutting away (or making lazily loadable by user request) everything except content. As website developers are not going to do it, it is better to do the optimization on client side. I wish standard mobile browser allowed disabling JS, web fonts (which are just a waste of bandwidth) and loading images on request. Mobile networks usually have high latency so reducing the number of requests needed to display a page could help a lot.